If there is a subwoofer line level output then that would be the best idea.The best way is running it directly off of the headunit.
Regarding polarity, I haven't open up my door speakers yet but doesn't the speaker terminals provide symbols for the polarity? Are the stock speaker wires color coded for each speaker with corresponding black line for (-) ground?I did this recently on my 2017 Mazda GT by tapping into the speaker-level outputs on the amp under the passenger seat. You have to test continuity and polarity, and connect a high-to-low level converter.
Continuity Testing
To figure out which wire was which, I tested continuity with my multi meter. If you haven't done that, just touch the metal tips on the multi meter and spin the mode knob until you hear a beep. Once there, you're in continuity testing mode, which will make a sound when the metal tips touch directly or share current through a wire. Now take off the door panels to access the speaker plugs on the door subs, and also disconnect the wire plugs on your factory Bose amp. You'll need to stick the multi meter's metal tips into the holes of the plugs on the door speaker and the amp.
With that done, stick one of the metal tips in one of the two holes in the speaker wire connector plug for the woofer on one of your doors. Then stick the other metal tip in each of the holes in the wires connector plugs that plug into the factory Bose amp. Just keep trying the next hole on each of the three amp plugs until you hear your multimeter's noise. When you hear a noise, you've found one of the two wires that runs form the Bose amp to the door sub. To find the door sub's other wire, move the metal tip in the door from one hole on the speaker wire connector plug to the other hole in that same plug for your door woofer. Then repeat the same process of testing each hole in the amp connectors until you hear the continuity beep.
Polarity Testing
Now you have both wires and just need to test polarity to find out which is positive and which is negative. To do that, first set your multi meter to DC Voltage (which sometime say "Vdc" on the multi meter dial), plug the connectors back into the factory Bose amp, turn on the car, and play the radio. Now stick each of the metal tips from your multi meter into the speaker wire connector for your door woofer; that is, just shove the tips into the door connector's two holes - doesn't matter which way you do it at this point. Whatever number your multi meter reads, it should be positive. If there is a negative number, you polarity is swapped (i.e. negative and positive mixed up). You will see the number switch from positive to negative when you swap the multi meter's metal tips in your door speaker's connector. When you have a positive number on the multi meter, the wire touching the red connector on your multi meter is positive, and the wire touching the black connector on your multi meter is negative. Now you know which hole in the door connector is positive, and which is negative. But you'll also need to figure out where positive and negative are at the factory Bose amp.
To figure that out, you have to go back and test continuity on one of the wires again, either positive or negative (keep in mind which you pick), to see which hole on the Bose amp's wire plug matches. Then do the other wire. At this point you should know positive and negative at the door and also at the amp for one of your two door speakers. Now do all of this stuff again for the other door speaker.
Connecting a High-to-Low Level Converter
With correct polarity information for both of your door subs, its time to hood up the high-to-low level converter so that your amp will have signal. To do that you you'll have to tap into the speaker wire signal. If you still want the Bose amp to power the door subs (assuming you do), you can't cut the wires coming out of the amp. Instead, use https://www.amazon.com/dp/ (commissions earned) connectors. They let you attach a new wire to the existing connections running from the amp to your door subs.
With a posi-tap in place and providing new wires for the four wires running to your door subs, use those four new wires to connect a line-level converter. If you're just connecting a sub, the best high-to-low level converter I know about is the https://www.amazon.com/dp/ (commissions earned), which is only $65 on Amazon - worth it for clean signal. Plug your left and right door speaker wires into the LC2i, connect the power, and ground (no need for a remote wire because it uses the speaker wires to turn on), and the LC2i will give you high-quality bass signal out of its RCA connectors.
Now just run the LC2i's RCAs to your sub amp. One of the cool things about the LC2i is that it gives you a remote wire so that you don't have to tear apart your dash to get one. Just run it from the LC2i to your amp. You can be done at this point, but I also added a bass EQ between the LC2i and the amp because the Bose equalization has crazy peaks around 50 hz. It'll sound decent with an EQ, but there's no way to make it really smooth without one.
Good luck!
Are those high pass (meaning it filters signals below 40hz) active or passive?Agree with headunit tap before amp. Bose has a high-pass around 40Hz on those little subs...
I did this recently on my 2017 Mazda GT by tapping into the speaker-level outputs on the amp under the passenger seat. You have to test continuity and polarity, and connect a high-to-low level converter.
Continuity Testing
To figure out which wire was which, I tested continuity with my multi meter. If you haven't done that, just touch the metal tips on the multi meter and spin the mode knob until you hear a beep. Once there, you're in continuity testing mode, which will make a sound when the metal tips touch directly or share current through a wire. Now take off the door panels to access the speaker plugs on the door subs, and also disconnect the wire plugs on your factory Bose amp. You'll need to stick the multi meter's metal tips into the holes of the plugs on the door speaker and the amp.
With that done, stick one of the metal tips in one of the two holes in the speaker wire connector plug for the woofer on one of your doors. Then stick the other metal tip in each of the holes in the wires connector plugs that plug into the factory Bose amp. Just keep trying the next hole on each of the three amp plugs until you hear your multimeter's noise. When you hear a noise, you've found one of the two wires that runs form the Bose amp to the door sub. To find the door sub's other wire, move the metal tip in the door from one hole on the speaker wire connector plug to the other hole in that same plug for your door woofer. Then repeat the same process of testing each hole in the amp connectors until you hear the continuity beep.
Polarity Testing
Now you have both wires and just need to test polarity to find out which is positive and which is negative. To do that, first set your multi meter to DC Voltage (which sometime say "Vdc" on the multi meter dial), plug the connectors back into the factory Bose amp, turn on the car, and play the radio. Now stick each of the metal tips from your multi meter into the speaker wire connector for your door woofer; that is, just shove the tips into the door connector's two holes - doesn't matter which way you do it at this point. Whatever number your multi meter reads, it should be positive. If there is a negative number, you polarity is swapped (i.e. negative and positive mixed up). You will see the number switch from positive to negative when you swap the multi meter's metal tips in your door speaker's connector. When you have a positive number on the multi meter, the wire touching the red connector on your multi meter is positive, and the wire touching the black connector on your multi meter is negative. Now you know which hole in the door connector is positive, and which is negative. But you'll also need to figure out where positive and negative are at the factory Bose amp.
To figure that out, you have to go back and test continuity on one of the wires again, either positive or negative (keep in mind which you pick), to see which hole on the Bose amp's wire plug matches. Then do the other wire. At this point you should know positive and negative at the door and also at the amp for one of your two door speakers. Now do all of this stuff again for the other door speaker.
Connecting a High-to-Low Level Converter
With correct polarity information for both of your door subs, its time to hood up the high-to-low level converter so that your amp will have signal. To do that you you'll have to tap into the speaker wire signal. If you still want the Bose amp to power the door subs (assuming you do), you can't cut the wires coming out of the amp. Instead, use https://www.amazon.com/dp/ (commissions earned) connectors. They let you attach a new wire to the existing connections running from the amp to your door subs.
With a posi-tap in place and providing new wires for the four wires running to your door subs, use those four new wires to connect a line-level converter. If you're just connecting a sub, the best high-to-low level converter I know about is the https://www.amazon.com/dp/ (commissions earned), which is only $65 on Amazon - worth it for clean signal. Plug your left and right door speaker wires into the LC2i, connect the power, and ground (no need for a remote wire because it uses the speaker wires to turn on), and the LC2i will give you high-quality bass signal out of its RCA connectors.
Now just run the LC2i's RCAs to your sub amp. One of the cool things about the LC2i is that it gives you a remote wire so that you don't have to tear apart your dash to get one. Just run it from the LC2i to your amp. You can be done at this point, but I also added a bass EQ between the LC2i and the amp because the Bose equalization has crazy peaks around 50 hz. It'll sound decent with an EQ, but there's no way to make it really smooth without one.
Good luck!
Oh I have misunderstood your post regarding small sub. By that you mean the door woofer. I never considered that a sub. Yes it gets full range signal.Highpass - I get signal below 40 hz through the door subs, which you can test yourself using the Signal Gen app (iOS) playing through the USB input on your car. Plus if you get the LC2i, you can adjust how much bass rolloff to eliminate.
Wire polarity marks - The wires are color coded, but it's all sorts of random colors and the colors at the speaker aren't always the same colors at the amp. You'll see light blue, dark blue, brown, purple, red, black, etc. If you want to save time, you can buy access for $20 to download the wiring diagrams from Mazda Service Info. There's no simple way, though. You're not going to find the standard "red for +" and "black for -" on the Bose/Mazda setup.
I'm actually planning to do a detailed post soon about my full install. It's only the bass right now, but I just got a DSP and a few more amps to power the signal. If I do a full writeup, which I've done in the past for other things, I'll include any diagrams I use.