door lock actuator. they are not cheap, over $100 a piece IIRC. all of mine were bad when i got my jeep. i went to the junkyard and used a drill battery to test them. i think i probably tested a couple dozen until i found 4 good ones. and heres another tip, you can use an actuator from a rear door for the front door too, you just have to swap over the little piece that goes to the key lock cylinder and cut off the little child latch thing.
Reliability is relative