The airhorns I installed are available in most autoparts stores for about $30 or from JC Whitney's. If you get them from JC Whitney's, refer to catalog #621J, page 130, item #12FV8359Y, price $19.99. The kit consists of a compressor, tubing, a relay, and two blue-colored plastic horns.
I opted to first remove the stock BMW horn, one of the most pathetic noise producing instruments ever fitted to a vehicle. Just cut the wires as close as possible to the horn. Attach crimp-on spade connnectors to the ends of the wires.
Next, you need to cut a bracket (green object in photo above left) out of metal in order to attach the horns and the compressor. This bracket will be attached to the frame using the screw and clip-on nut used by the original horn. I will provide a template for the bracket soon, but the twist that needs to be imparted to the bracket is something you have to experiment with. The compressor rests on the bracket and is secured with two thick plastic ties. As shown in the above right photo, the horns attach to the rear end of the bracket. home