Is the aux fan the only thing on that circut? I'm not familiar with your model, but on my 1986 420SEL, most of the circuts run several different things, most of the unrelated to each other and at different ends of the car. Check to see if anything else is running on that circut, maybe that is blowing your fuse. Other than that the only thing that should blow your fuse that violently (if it is not your fan) would be if the wires are borken or bare somwhere and the live is in contact with either the ground or neutral. This could evn be inside the switch maybe.

JP