I bought the replacement actuator motor on eBay. With success. I did both rear doors (they went bad within a month of each other).
The motors are located at the very rear of the door, under the door lock mechanism. They are held in place by two or three rivets that you will need to drill out and replace with bolts.
These don't go out without a reason, I learned. Both my locks were pretty dry and very hard to operate. So I pulled them out too, cleaned them perfectly, lubricated and now I can lock/unlock the door with my pinky.
I'm sure somebody else will chime in here with more info and possibly a link.