If the horn works without the relay, but doesn't work with the relay, and you've tried multiple relays, then it must be the way you are wiring up the relay. If you have a multimeter and know how to use it, it's time to do so. If not, I suggest that maybe it's time to find someone that does.

I know my way around a meter and wiring, but I can't guess at which of the many ways possible, that you may have connected the relay incorrectly. Sorry,

The test light shows power on terminal 30, but none on 86 & 87 when the horn button is pressed, but now the light on the wire to the (+) terminal isn't coming on, and the relay isn't clicking. Will check back with you when I figure out why!

OK, back to normal. Power on terminal 30, none on 86 or 87 when button is pressed, but relay clicks and light comes on. Clue?

Love my Stebel. Comes with a built in "texting" detector.

Something is hooked up wrong.....its the only option. Otherwise you horn would be working, or you would be melting wires/blowing fuses.

I don't know what else to tell you other than to ignore whatever instructions you have and wire it like I initially diagrammed. I don't know the supplier, but are they used to dealing with BMW electrical systems? This might not be the same as hooking it up to some random Japanese bike or Harley. I have an assumption that the horn circuit is tied into the CANBUS system somehow, and that original horn (-) connection isn't the same as battery (-). You might be causing some type of screwy ground loop.

Relays are pretty simple things. You apply 12V between one set of terminals (85 & 86), which in turn connects another set of terminals (30 & 87). If I was standing there, I could troubleshoot it in a matter of seconds, but its obviously a little more difficult like this. If you want to eliminate the horn button circuitry, just disconnect the wires from 85 & 86. Grab a good old 9V battery and stick it between 85 & 86. That should energize the relay and cause the horn to blow.

Is the light wired in series or in parallel??? It should be in parallel to the horn.

I think, but will double check, that everything is per your instructions EXCEPT that there's no wire to the (-) wire to the original horn button.
I'll wire that in this evening and see if it fixes the problem.
The instructions that came with this kit didn't call for it, but it wouldn't be the only error I found if it turns out it's wrong.

With work and the holidays, I just got around to adding the wire mentioned above. It didn't make any difference. I am totally stumped.

Don, wire it like this,, don’t worry about the resistor shown as that is for more advanced installs.. Remove the original horn..

If you wire it like shown & the horn is good & the relay is good it WILL work..



Twisty

Well this is too stupid for words. There was no fuse in the fuse holder, but the LIGHT on the holder came on consistently, so I never even thought to look. I figured if there was ONE wire that was right, it was the one from terminal 30 to the battery. I KNEW it was going to turn out to be something ridiculously simple...
After making a wiring change, the light stopped working and I thought I might have blown the fuse, checked and found.... nothing. Why was the light coming on if there was no fuse?
So now I need to know what size fuse belongs in the holder. I can find no clue with my instructions (which also don't tell me to install a fuse as part of the install).
Thanks for the diagram. I understand the circuit a little better at the very least, and the diagram would have been easier to follow than the poorly written instructions. BTW, though, my instructions didn't call for a wire from the (-) side of the original horn wire, and the horn works without it. Don't know why...