My 84 300D did the same thing. Check the kickdown switch wire leading from the transmission up to the inside of the car. My leaking trans. (410,000) had eaten the plastic electrical wire jacket and the wire was shorting against the frame. Whenever I engaged the kickdown switch the fuse would ground and blow. I just cut the wire, spliced it and used a wire nut. Not pretty but that was a year ago and it stil works.