I'd be checking them grounds again... Keep in mind that the wires to the horn are ground wires. The positive wire is not switched. Pressing the horn button completes the ground circuit, not the positive.

I have a similar problem with my '65 1800... though...affecting different lights. On mine, if the horns don't blow when I push the button, a light under the dash comes on. The culprit seems to be one of the purple wires with the black stripe. More specifically, the connection of those wires under the hood near the driver's side headlight.

Check, double check, and triple check every connection that those wires make. And...also check the fuse. As I recall, the horns are for some reason the only thing on that fuse, which is in a separate fusebox. There might be two fuses in there, but only one does any work best I can tell.

Have you tested your horns to see if they work at all? Unplug the wires from the horns themselves and ground the connectors? If they do work, go to the next connector back and see if they work from there.

Personally, that horn wiring on the 1800 baffles me a bit. Been there many times and I still don't quite get it. But....I'd put money on the purple wires with teh black stripes. That is...if it is still original wiring. If someone has monkeyed with it in the past....well...

in that case, best to isolate the horn circuits and start from scratch. Shouldn't be too hard to do that...

Good luck, and keep us posted!
-Matt
And...keep in mind you are now dealing with the beloved Lucas electrics...



Good luck, and keep us posted!