I am electrically challenged and I am trying to add a set of air horns to a 1988 744Ti.

The lead to the existing horns is grounded when the ignition key is off. That lead breaks the ground when the key is turned to the on position (which also enables the horns circuit) and with the key on when you push the horn button it once again provides a ground.

In trying to hook up the relay for the horn’s air compressor that is a problem. When following the directions for installation for a negative wire running to the existing horn the air horns blow as soon as they are connected because with the ignition off that wire is grounded. Turning the key to the on position turns off the air horn because the wire is no longer providing a ground. With the key on pushing the horn button causes the existing horns and the air horns to function as they should (until I turn the key off when the air horns are once again turned on).

Even tried hooking them up for Plan B (positive wire to horns) and it works in just the opposite way. Horns do not blow with the key in the off position but as soon as I turn the key to on the air horns blow.

I have installed these horns before without a problem. One set on a 240 GLT and another on a MB 380SL. On both I used a wire leading to the existing horns to activate the new relay.

Do the existing horns on the car have a relay? If so can you tell me where it is please? It would seem simple enough to find a pole on a relay that will either provide a ground or 12v when the horn button is depressed with the key on and just be an open with the key off. Thanks for any enlightenment.

Randy