...when you raise the rear seat...you should see the control unit on the drivers side. If the control unit is there...all you need to do is pick up a 3 button remobte fob...make sure it has a chip in it. When you receive the remote...take the chip out and place it in the control unit and hit the reset button. ( this codes the chip to that control unit) Once the chip is coded, place it back into the remote fob.
Click this link to see location of control unit under rear seat (of an e34)...AND...to see the instructions and details about the BMW Alpine keyless entry system
Instructions how to lift rear seat:
1. get in and straddle the tranny hump with your bum raised above the bottom cushion but your back against the upper cushion.
2. place your hands at the center point of each side and get a grip of the lower cushion.
3. With you back braced against the upper rear cushion, your bum raised above the lower cushion, and your feet on each side of the tranny hump...LIFT UP firmly on the lower cushion to release it from the clips on each side. (you can see where the seat clips into the car in the pics at the link above...there is a cut-out in the carpet along the ridge that the seat rests upon).