Dealer? E-Bay? Salvage Yard?
It depends on the vehicle.
When power doors fail on a 1999 Honda Civic there is a problem with a actuator or wiring. The door lock fuses should be checked. If the fuses are fine then the actuators themselves will need to be checked.
Bad lock actuators? try replacing the lock solenoid. there only about $25
Need removal and installation instructions for an ignition lock cylinder -- 1999 Grand Cherokee Limited?
Check for a broken wire in the harness going to the drivers door.
install an alarm system and door lock actuators..
www.autozone.com Click repairs, component location.
You can look into the Autozone online wiring schematics or the Hayen's Book
with pics! http://www.eurekaboy.com/f250/doorremove.htm
your door may not be closed all the way or the keys are still in or you did not lock it.
If it is a coupe there are two. If it is a sedan there are four.
Not any relays, there are actuators inside the doors that move the lock mechanism. Also your switch in the door, the fuse, and the wires.