There's a bolt that takes an 8mm hex bit going through the center of the fan clutch into the pulley. Unless you have a very stubby 8mm hex bit socket your best bet is to put an 8mm allen key into the bolt and use some pipe as leverage on the allen key. If the radiator is out of the way you should have room for conventional tools.

Looking from the radiator towards the fan there are ridges on the fan pulley stand at about the 1 o'clock position. get a length of 2-3mm diameter stiff wire and bend into an L with about an inch of leg. Fit the leg between the ridges and slide it forward. Turn the fan pulley so the leg engages one of the four holes in the pulley. This will lock the pulley so you can release the bolt.

Sixto
95 S420
87 300SDL