Various configurations of the M103 have different methods of holding the fan clutch in place. I believe the 300E has a couple of ridges at the 2 o'clock position on the fan stand behind the pulley. Put a hex key (4mm?) or a really stiff piece of wire bent to an L between the ridges and slide it forward to engage one of the holes in the pulley. You'll have to turn the pulley to find a hole. That should hold the pulley while you work on the allen head bolt holding the fan and clutch. If you have stubby fingers, try pliers to get the allen key in place.

My don't-try-this-at-home-kids alternative is to tighten and loosen the fan bolt with the belt tensioned.

Sixto
91 300SE
81 300SD