Since you just bought it, it is possible. One might of went bad, the owner neglected it, then the other failed. All speculation of course.

An auto probe is the best best to inject power into the plug of the actuator. You must first figure out your +/- wiring on the actuator harness.. If you don't have a probe, you can get creative with 2-wires from a 12v source. If that seems like a headache (especially if there's more that 2 or 3 wires on the actuator and you don't have schematics on it), You can also swap the rear actuator to the front, Obviously it will not mount to to the front door, but it should plug into the same harness. Try it on the passenger front door.