Can't get horn to work on 68 Barracuda

I've been trying to get the horn to work on the old 68 ragtop. The relay is inside near the fuse box and it seems to be working just fine. I even took it out and checked it on the bench. Anyway, when I ground the horn wire the relay kicks in, closes the switch and sometimes the horn blows (rarely) but sometimes the relay acts like a buzzer. I tried to jump V+ directly to the relay, no change. I tried to jump ground directly to the relay terminal, no change. I even swapped the relay with one from my 67 Barracuda, no change. Both of them work on the 67 car but I get no horn on the 68. If I jump the horn wire hot to the battery I get both horns just fine. What's going on here?