When I bought my car the dash lights worked but my horn didnt. Whenever I hit the horn the lights would dim just a tad but nothing would happen. I traced every wire from the horn ring on the wheel to the horns themselves and couldnt find a thing wrong. There were no frayed wires, shorts, and voltage was good at all the right places.
Just on a whim I bought a new horn plugged it in and it worked... Both of the original horns had failed completely and would not even make the slightest noise.
Do you get any indications like lights dimming or a clicking sound in the dash when you press the horn? Have you checked the voltage at the horn button and at the horns themselves to see if they are getting juice?
As for the dash lights they were intermittent. If you turned the headlight switch counter clockwise the lights would not progressively get brighter they would just turn on right before the interior courtesy lamps activated. But the dashlights wouldnt always come on I had to finesse the switch right at the turn on point for the courtesy lamps. It got annoying if I was driving at night and hit a bump the dashlights would kick back off.
I ended up having to replace the headlight switch. Before you replace the switch though it would be a good practice to check the fuse block and/or to pull out the instrument cluster to ensure that all the wiring harness plugs are in place and there are no cut wires etc. The previous owners of my Cougie were not very smart in the ways of electrical and there are splices and cuts everywhere, plus old wiring gets brittle and can deteriorate and break easily too.
Hmmm, my problem might even not be a problem at all.
It turns out the switch was turned clockwise all the way. If I understand correctly the dash lights should be operated by a dimmer switch in the headlight switch...
So I think I'll go and try first if that's it when I've got the battery and all back in the car.
Well, the lights turned out to be a problem in the radio wiring, or lack thereof. someone pulled out the harness and cut the feed wire. I fixed it. Now I have to tackle the horn. I think it may be the switch. It's a grant wheel and the contacts are all worn. I'll test it further today. Also just got the amp light to work with my GM alt. It was very simple and my 3 wire GM Alt install is now complete, so stand by for a write up.