Bro for your further reserch about the BIG red bulb/button for? Maybe you can refer this link :
http://www.bmwgm5.com/E46_Electrochromatic_Mirror.htm
This mirror has the auto dimming feature, Home Link garage door opener and 315 MHz remote receiver (FZV) for the key fob.
This is the picture of the back of the actual auto dimming mirror assembly. It's basically one big LCD display pixel.
HomeLink garage door opener board. You can just see the three buttons at the bottom.
Below is the FZV antenna amplifier which is only in the rear view mirrors used in Convertibles. Normally the FZV amp is in the drivers
side C-pillar, but, in the convertible it's in the rear view mirror.