Hey friends,
My sister has a 2012 Mazda 5 with the dealer installed remote start and wondered about this same thing.
After digging around on the internet a little we found the installation guide (Might be the same one linked here -- not sure). It showed a black+yellow behind the glove box that hooked up to the door ajar sensor.
So we took the glove box apart. Took the {glove box} door off. Didn't have to take the panel behind the glove box door completely out, just took it apart enough to peel it back a little and worked through this "slot."
As per the installation guide, behind the glove box there was a yellow+black wire. It was near a gray switch, which I also expected from the installer guide. The gray is the hood switch. I think there was a brown wire bundled in too.
Pulled on the wire and it had a lot of slack -- not like the factory wires that are all bundled down tight as can be.
Carefully negotiated some wire clippers into the area behind the glove box and clipped the black+yellow wire. Voila! Remote start doesn't know when the doors are open now. I can remote start the car with the doors open, and when I approach the car and open the doors, the engine keeps running.
Somebody said something about theft, but it still shuts down as soon as you hit the brake pedal. This is consistent with other remote start systems (Including Mazda's own, prior to 2012).
Capped both ends of the clipped black+yellow cable with a bit of electrical tape just to be extra extra cautious.
We haven't seen any ill effects so far but I'll report back as we do. So far we just see a remote start system that works the way it should, without this stupid probably lawyer-induced "feature"