IIRC, my 1985 300D kept blowing that same fuse. It happened as soon as I took it out of Park. It ended up being the wires that cross the back of the transmission. The insulation on the wiring wore through and it would make contact with the trans resulting with a blown fuse. Hope this might help.

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'85 300D 190K miles
running on B100 Biodiesel since 156K, SCRATCH THAT! I can only find Biodiesel at 1 location in Houston! (not very convenient)