The quickest way I know to isolate the problem would be to remove the door interior panel and check for power at the terminal connector for the motor on the door frame. If there is power and the motor doesn't turn, you need a new motor. If the motor turns but the regulator doesn't move, you've probably got a cracked regulator frame. This is common on older 123's. The regulator and motor can be replaced separately on this car, so don't assume you need both.
If you don't have power to the terminal connection on the door frame, obviously trace the wiring back to the battery to find where the fault is located.
I have replaced regulators and overhauled motors, so I'm at leart speaking from some experience, but I'm by no means a pro.