exactly , the original bolts that hold the horn , and other electrical components throughout the car , sometimes have toothed washers that will dig into the thin paint on the part . This is how they get their ground instead of using a seperate wire , the washer digging into the paint combined with the bare threaded hole that the bolt screws into makes the ground connection.

With the practice of over restoration that is prevalent in the hobby many parts of the body get painted that were never originally painted, in combination of over restoring parts that get bolted on .,. kinda like powder coating a horn ..... can create electrical issues ...