Can someone give me an idea of what causes distributor rotors to crack and brack? 1966 F100 with 352 engine. We have gone through 3 rotors within 3 months. Thanks

I am having a problem getting the horn to work on my 66 F100. I was wondering if someone could tell me how the horn relay works. I tested the horn and it does work and am trying to work my way back to the horn contact spring under the steering wheel. I have three wires on the relay, Yellow...