On most cars there are two wires running to the electric window motor and both are positive. One tells the motor to go up and one tells the motor to go down. The motor is grounded through the body of the car.

On Mercedes of this era the motor direction is determined by the poliarity of the wires. That is: when the the window is to go up one wire is (+) and the other is (-). When the window is to go down the poliarity reverses and the motor turns the other way.

You can also dimantel these motors, clean them, and then re-grease them and put them back togetther. This is not hard but it is time consuming.

Before removing the window regulator assembly test the voltage to the motor and see if it is getting there. Put the (+) wire of your volt meter to one of the wires running to the motor and the (-) wire of the meter to an unpainted part of the car (like, in this case, a screw holding the door sill on) and move the window switch. After this you will know which way to work towards your problem.

Pooka