since you have a 10 series, try messing with this:
http://www.ft86club.com/forums/showthread.php?t=11049
IIRC there is an option in that set up where you can choose for the car to either 'beep' or honk the horn after turning the 'alarm system' on. I'm just recalling off the top of my head, I'll try it in my car after work.