A grease monkey is slang for mechanic 
You would need to determine which door's actuator is at fault (a mechanic can help you here if you don't know much about cars or don't want to go pulling door cards off), then go looking for that particular door's actuator at a scrapyard. You will need to look for the right model part as the year of your car, and if it is for a three door or four door. You won't be able to use an actuator from a three door to fit in a four door model. Well, in my experience at least, that was the case. As for finding out which actuator is causing the problem a mechanic should be able to tell you once they have the door cards off and lock/unlock the doors repeatedly. In my case the faulty door actuator had completely stopped working so it wasn't difficult figuring out which one was faulty!
Hope that helps
