power door locks problem

I have used up 8 power door lock actuators in my 1999 F250 SD, the first 4 under warranty, the next ones out of my pocket. I have checked the wiring and both switches and they seem to be OK. Two things still puzzle me 1. what can cause this kind of problem 2. Where on earth are the relays for the door locks. It isn't under the hood with the rest of them. Can anybody help???