How can I fix my electronic door lock for my '92 Ford Taurus?

The electroninc door lock in my '92 Ford Taurus is not working. The 'lock' option of the passenger's side works, but the unlock does not, and neither the lock or unlock buttons work on the driver's side. The key will still unlock and lock both front doors, but the rear doors have to be manually lcoked and unlocked. How can I fix this problem, and what exactly IS the problem?