The horn on my 1952 1/2 ton doesn't work when the engine is at idle. At highway speeds it works great. The truck is 6 volt. I have replaced the horn. I have added a horn relay. I have replaced the wire going up the steering column to the horn. My guess is, like most of the electrical problems with this truck, it is related to a bad ground. My understanding is when I press the horn button I am completing the ground connection (now that I have a horn relay this causes the relay to close). I'm guessing at idle the voltage drops enough to not close the relay.
How can I figure out exactly where the ground is failing? Is there something in the steering column that gets dirty/rusty that I can clean? Or add a ground wire to?

My dad always told me, "Don't force it, get a bigger hammer".