65 Dart Horn Problems

No, looking at the backside of the steering wheel you have not lost an insulating part. The insulation I referred to in an earlier post is that shriveled up, white looking crud, under the looped round steel retaining ring. Either enough of that is missing and allowing that looped retaining ring to touch the horn contact plate or the horn contact plate is bad which I see as less likely. I had not given the idea that you have an optional wood grain wheel much thought. I'm more familiar with the plastic coated steering wheels. I say this because the wood grain wheel may also have a similar insulating goo between the horn contact ring and the steering wheel body that if not there or bad would allow it to ground out. I'd consider removing the looped steel retaining ring (it's also you turn signal canceller so mark it so you can put it back on in the same place) and inspect/replace the insulating material on both sides of the horn contact ring. As cudamark suggested the horn contact ring, while resting on the steering wheel and held in place by that looped ring can't actually touch either, as in metal to metal contact.