Try turning the aux fan by hand and see if u feel any roughness in the bearings, I had the same problem where the fuse would blow any time the fan came on. It turned out to be the bearing in front of the fan, it was rusted and preventing the fan from turning freely and thus the higher current draw would blow the fuse, if thats the problem the don't blow $150 on a new fan just buy the bearing mine cost $8 and replace it. My car has 244k with the original fan and the brushes on the fan still had about 1 inch of "meat" in them.
It can also turn out that you have a problem on a diff circuit that's covered by the same fuse. Do the fan first and thats one less circuit to diagnose.

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87 300d Turbo 241K with original #14 head (blue)
87 300d Turbo 198K will run great again someday (silver)
84 300sd sold ( buyer drove it to Paraguay )