There is a peach parts wiki paper on changing the fan clutch. It is here:

PeachPartsWiki: R&R of the Viscous Fan Clutch

The paper shows the special tools you need -- an 8mm stubby socket and a heavy wire tool to keep the water pump from spinning. Samstag sales has the tools. Mercedes has changed the bolt that holds the fan clutch from the 8mm hex head to a torx head (I think T55). The new torx head is engineered to also accept and permit use of the 8mm stubby socket.