My 1999 Ford Explorer front passenger door intermittently locks.

I have a 99 where the door will automatically lock once you begin driving. If the door does not lock when the button is pushed you may have a bad contact at the switch. Remove the switch panel from the inside trim assembly and check for voltage at the switch terminals for the lock with the key in the "on" position. If none present you can check the wires inside the boot where wires pass from the door into the body of the vehicles. Sometimes you get a break or wear at that location from the door being opened or closed.