Mine is a 74 450 SL, if yours is the same then remove the door panel. You will see a terminal strip with two wires connected. Look for 12 volts there when the window switch is activated. Warning...what stops the window from going up too far or down too far is a mechanical stop, so power is still applied to the motor and it will continue to try to drive the window even though it has hit the mechanical limit. So clip your voltmeter to the terminal and only run the window long enough to see if the 12 volts is present. If present but no window movement, you need a new motor, or the wires from that terminal to the motor could be broken. If no 12 volts present then the switch is bad or wiring from the switch to the motor terminal block.

Look on the message boards for info on removing the door panel, there is quite a lot of help available...........Dan