I had the same problem , it was my brushes in the motor they had worn down. Hope over to a Radio Controlled car hobby shop and pick up some brushes from an RC motor ($6). There will b 2 brushes on a wire make sure you get the ones that have the wire coming out the back and not the sides. Now the brushes will be a bit big but thats no problem as you can sand it down to fit inside the housing. Remove the screw that hold the motor in place and unclasp the 2 plastic clips that hold the motor in place. Angle the motor at an angle and unhook the springs that push the brushes to the armature. pull out the brushes, unscrew the 2 screws on the bottom and remove the magnetic housing the armature (large motor looking part inside the housing) the top is where the brushes make contact with the armature sand it down so it is clean and shiny again, blow out the housing to get rid of any dirt inside. Now clip the wires and measure the wires so it will fit snug in the housing and sand down the brushes so they fit smoothly in the top housing, solder the wires together, not too much lead as the wire eats up lead and will get still if you put too much. now to assemble: (the tough part) slide the armature in the housing and let it sit at the bottom half insert the brushes with no springs at the plastic top, insert the top and bottom together till it sits flush the housing only goes in 1 way so check the alignment of your 2 bottom screws if its not flush you have it on backwards. Now screw the 2 halves together and insert your brushed all the way (don't insert them all the way till you have the armature in and the housing screws together or it will snap the brushes and you have to start over with new brushes) once the brushes are inserted put the retaining springs back in make sure the brushes touch the armature top and screw it all back on the door and put the clips back. Now test your door , it will be noisy but it will work for a few years as the brushes are for high speed motors.
Good Luck
Fernando