1999 gmc sierra 1500

http://www.vadengmpp.com/owners-manual/gmc/2003-GMC-Sierra.pdf

Missing Egr Valve 1999 2006 2007 2013 Chevrolet Silverado GMC Sierra 1500

the split ring in steering column is a ground to horn relay if wire touch ground or pad is compress horn will be active
hope it helps

On a 1978 the fuse should be in the box under the dash if it is not a circuit breaker.

Your horn should be located on the front passenger side behind the passenger splash guard located in the wheel well. it is well out of sight from the engine compartment. You should be able to get under the car and see a conch snail shape held by a bracket on the other side of the passenger splash guard covered by the bumper.

Sounds like the horn itself is shorted out. You didn't mention if the blown fuses were a result of using the horn. A horn in construction is an electromagnet acting on a diaphram; if the electromagnet develops a short, it causes an overcurrent condition and will blow the fuse.

Either the horn itself is bad or the horn switch is bad in the steering column. Try running a hot wire straight to the horn to c if it works

The switch in steering wheel may be shot. Also check wiring to horn and you can also try connecting horn straight to horn.

The horn is located just ahead of the washer fluid tank in the L.H. fender. Access is by removing the extra battery tray if you have one, and then the washer fluid tank. There is a pump wire, fluid line, and a fluid level wire to be removed before the reservoir drops out the bottom ahead of the wheel well liner. Best to drain the washer fluid before proceeding.