Step 1: Are You Ready? Do you have all the parts? You'll need the control unit ("black box"), the cable with a 12-pin main connector, a 1-pin hood switch connector, the LED and its leads, and the siren. I bought some industrial-strength Velcro to secure the "black box" and used an M5 10mm bolt with locking nut (left over from the window regulator install) to install the siren. I used a few basic tools: Phillips screwdriver, a socket set, drill + 7/32" & 3/4" bits, rubber mallet, and a pin punch.
Step 2: Prep Call me paranoid, but whenever I plan to monkey with automotive electronics, I disconnect the battery when possible.
Photo 1: I disconnect the battery.
Step 3: Installing The Control Unit The control unit is mounted to the bottom of the electrical carrier, found behind the glove box. There are nine Phillips screws (or eight if you're missing one!) holding the glove box/lower trim in place.
Photo 2: There are two screws along the left bottom edge of the trim panel below the glove box.
Photo 3: There are two screws along the right bottom edge of the trim panel below the glove box.
Photo 4: There is one screw along the side facing the passenger door of the trim panel below the glove box.
How To Install A BMW U.S. Dealer-Installed Alarm 
Threaded Mode