Door lock that functions with passenger-side switch but not driver-side? It's likely the driver-side switch.
Door lock that functions in only one direction? It's likely the door lock actuator.
All door locks function in only one direction? It's likely the door lock switch.
Motor failure from wear-and-tear and carbon buildup
Sustained exposure to long pulse radio frequency
Damaged cable, which can damage actuator housing
Internal circuitry failure from normal operation (3 years/36,000 miles)
Lock Cylinder replacement for Ford Trucks (E-Series Vans (2005-97), F-Series Trucks (2010-96), Explorer (2010-96), Ranger (2003-96), Windstar (2003-99), and Expedition (2010-97))
Approximate installation time: 1 hour
Tools needed: T20 Torx driver, flat blade screw driver, 90° pick, straight pick