Thanks to the OP and all who have contributed. I've used this info today to convert my 2004 non-Bose Mazda 6 touring to have an aux input using buba's blank CD method with Dragan's trick of soldering direct to the PCB. Reminder - do not try to short pin 13 on the back of the radio on a pre-2006 car! I'll add some pics and notes from what I learned, in case it helps anyone:
First pics of the stereo opened up
Lower PCB board (under the CD player)
Upper cd board (attached to underside of CD player)
Upper board after wires soldered:
Pinout is the same as the (labelled) connector on the lower board. The ribbon cable is direct pin to pin after all. So from left on the top row you have audio right, audio left. On the bottom row the LHS pin is audio ground. Do use hot glue to anchor the cables before you solder them!
Ok, lessons learned.
- First, the tip to refit the gear knob is a good one, it does save scratching your trim
- Second, the cup holders on the 2004 car don't have the plastic tabs referred to in the OP, you simply grab the whole unit including fascia and pull it up hard. Then squeeze the two tabs which lock it to the next panel (gearshift fascia) and release them
- it feels so wrong to start pulling your car apart but I suffered no snapped clips or damaged plastics. Trust the instructions and use a bit of grunt
- when removing the centre trim you need to unplug at the rear of the radio and the side of the radio, but the lower connectors can stay attached. You can unscrew and remove the radio without taking them apart.
- taking the top off the radio doesn't require removal of any screws, just prize it off with a flat blade screwdriver
- a magnetised small crosspoint Philips screwdriver will be very handy for removing the CD player, as the two screws at the front are buried deep
- remember to thread the audio cable / cables thru holes in the case BEFORE you hot glue and solder them
- I soldered to the top board (the one which is attached to underside of the CD player) and found this much easier than the lower board, as it's more accessible. Even though the lower board is the one which is labelled, the connections do of course make it through that ribbon cable in a straight line, so you can see easily enough where they appear on the top board.
- use hot glue to hold the cable close to the solder connections, and more to anchor the cable as it leaves the PCB. Use a cable tie to strain relieve the cables as they pass through the radio case. You don't want to be taking the radio apart and resoldering because someone dropped their phone and gave the audio cable a tug.
I think that's it. It was a lot easier than I expected. Only hard parts were getting started on taking out the cup holders, and persuading my old windows laptop to burn a blank audio cd!