There is no reason to go BACK to the $hitty IR system.
And the car shouldn't have IR alarm in it anyway. Those were el ghetto craptastics, and they were history by 92... my 3/92 had rf already. if yours does have one, then you have the ir reciever on the dash already. The only possible reason to want it is the switchblade remote...
Wiring the mirror in is easy. check bf.c, I posted all the colors and pins up a while back. on e38s some have the sensor and some don't. most others use the clown just for led.
you have a 3 wire mirror harness. anything else (led for example) will be custom wired by you...
http://forums.bimmerforums.com/forum...2#post10211022