[eeyang92]

Small update:

After taking apart my steering wheel and inspecting the PCB, I came to realize that there are 9 buttons (or so) but only 8 pins on the PCB. This implies some sort of multiplexing - if I had to guess, half the pins are routinely sent a signal and the control board watches for a short circuit on the other pins (which is achieved when one of the buttons is depressed). 4 pins allows 4x4=16 combinations so this seems to match to my theory - however, I did not measure the voltage with the car battery connected because I didn't want to break anything.

Anyways, all this means is that I cannot wire the M1/M2 buttons directly to my remote (which contains it's own battery) as I would effectively be putting these two circuits in parallel, and be constantly sending current into the control module, which is definitely not good and will cause unexpected behavior from the entire cluster.

However, I do have an alternative solution that I will share if it works - parts I need will take a while to arrive, but I'll update here when that happens.