It's a black box about 3/4 inch thick and (if I remember) about4" x 5". There is a short (6" ?) wire that sticks out of the box...this is the antenna. There are a couple of switches on the bottom to turn on/off alarm sounds, etc. (These buttons might be behind the door on the bottom of the box, I forget). Inside this door, you can place your chip from your fob and re-program it.
I'm assuming that you have an original BMW alarm system and that you have a three-button fob (I'm taking this from the model year of your car).
<span style='color:green'>1994 325i </span>- - - 75,000 mi.
No mods - - - very happy as is!
<span style='color:green'>1995 525i </span>- - - 154,000 mi.
No squeaks or rattles... unreal!