Not sure if entire system works,car is 1/2 apart,however the horn worked when I hit it with 12V battery.I also looked uder fender where a key/logk/tumbler deal would have been mounted thru fender and saw no bondo/fibreglass repair as if it were removed.I guess this 77 did not originally have horn under rear fender,besides hot wire is up front as well.

Reread my response and see I wasn't clear. Drivers side FRONT inner fender.   Mike