I swear to God this is true. Had a '61 Chevy with a 6 cylinder. 2 days after we bought it from the original owner ( who said he didn't want anything that wasn't a Ford in his driveway and since his son had left it in the driveway when he got married, he was selling it since it was still in his name ) we noticed that the battery was hooked up backwards---yes, backwards---negative to the starter and positive to the block. We decided "What the heck, let's turn it around", and when we did, the horn began blowing and would not stop until the relay was replaced. This is no remedy to your problem, but I was reminded of that incident when I read your dilemma. Most likely, you have a tilt steering wheel, and it is quite common for wires to get chaffed in those. Happy hunting.