Hans - The blower motor has a pre-resistor, a separate unit and it is located somewhere between the 3-way switch and the blower motor behind the dash on the fire wall I am told. Can't tell you exactly where as I looked for and could not find it and later found my problem was brushes. No need to remove the 3-way switch - follow the cable from the blower motor - you may have to pull the dash foreward.
Have you checked your brushes? When they wear right down they will deposit a copper ring around the armature which will blow your fuse.

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Beagle