I have done the definitive test for running 6 volt horns on 12 volts. They love it! The pair I am using now was on a 92 Dodge for 5 1/2 years before I moved them in 1998 to a 97 Buick. I use them regularly and even for weddings. They are as dependible as any other P-15 part. They must be wired with #10 wire from the original relay which is excited by a splice from the cars horn wire. Polarity is not a consideration.
Before that, I used P-15 horns on an 87 olds and a 76 Newport, but I don't know if it was the same pair I am using now.
The point is, they work perfectly, continuously, dependibly, and boy, are they loud!