Ever since I've owned my car, the rear driver door has never locked electronically with the others. It'll lock fine on its own, but having 3 doors lock at once, then locking the 4th seperately has become a pain. How do I go about fixing this? What should I be looking for as probable causes?

Switches and connections are the most likely culprits. Check and clean both, test with multimeter if available to determine where the issue is.

Agreed, also may look into the lock motor. I personally haven't heard of them going out....but I'd imagine it'* happened before.

Does the lock TRY to lock or just does nothing?