I just did a repair on my rear window sliding JAW last month so maybe I can help here.
First, I'd check the switch in the front center console to be sure that it works ok. these usually wear out from 'gunk' seeping onto the contacts. You need to remove the ashtray, then there are two phillips screws to remove before the center console loosens to the point you can lift up the panel that holds the switches. If I recall correctly, the front and rear switches are identical. I popped mine out and gently opened the switch and cleaned it before I enventually replaced it.

To check the door switch you will need to remove the doorpanel. Remove the door lock knob, gently pry the black plastic piece inside of the door latch to expose a small phillips screw. Now there are two large phillips screws in the bottom of the door arm rest to remove and another at the top of the door rest handle. Also, there is a trim piece if I remember that goes up the side and is held in by two small screws. Once all of the screws are removed, GENTLY tap the bottom of the door panel upwards and the panel should loosen to the point where you can slightly pull it out and reveal a light connection in the bottom (remove) and the window switch harness (to be removed so the panel can be completely removed) DO NOT try to pry the door panel straight out as you will break the plastic pieces that hold it in place.

Now you can check the switch, clean it and/or replace it if needed and you can carefully cut the plastic liner and check the motor and JAW action. These motrs rarely go bad and I have yet to replace any in the three benz's I have owned.
Good Luck!
I bought all of my parts from fastlane. they are GREAT!:p :p