door lock relay

sometimes the door locks work perfectly, sometimes not at all, and sometimes they will lock but not unlock. They would work until you opened the door, and the switch to turn on the light came on and all would quit. I disconnected the switch and all was well for about 5 minutes and back to line one of my descr. Is it the relay, and if so where do iI find the door lock relay on the car so I can replace it?