[Drbrown]

I gather you do have two horns so I also think you have a grounding problem. The horns must be grounded to the car body. No big deal but did you change the relay to a 12 volt (i.e. a 1956 Ford). Regarding voltage, some don't worry about 6v vs. 12v if there's just incidental usage. Its continuous and/or long-term usage that could cause damage.

Also, if you're using two horns, wiring them in series creates some conflicting electronic problems as the vibrators in each tend to fight each other, usually to the detriment to the second horn in-line.