How do I adjust head lights on 1979 Mercedes Benz 300 D?

The main and fog lamps have two adjusting knobs each, they are located on the rear of the headlamp housing. They are black plastic thumb-screw knobs with four points, approximately two inches (5 cm) long. Top ones affect vertical aiming lower ones control horizontal aiming. The knobs are readily visible and easily accessed (driver's side requires removing windshield washer reservoir). Turn the knobs slowly and carefully to avoid stripping threads. If lamp will not adjust, this condition may indicate corrosion in the headlamp housing, necessitating removal, disassembly and cleaning.