I have the same starting issue with my 1985 300D. I've done a lot of work on this. New shifter bushings, tested NSS, just to mention a few. I did not know about the headlight switch possibly being the culprit, but I wiil check it. I still have the intermittant problem.
BrassMonkey, I tried running two wires to a switch in the cabin, but obviously, I needed some real heavy gauge wire. It draws way too many amps. I think I used 12-14 gauge wire. It wouldn't do anything.
I wrote to a guy on here who had a post about installing the Ford starter solenoid, and I think that's what I'm going to do. He said to use one from a 1965 mustang or something. I certainly don't want to spend the money on a new starter and end up with the same issue. Not to mention what a pain these starters are to swap out.
You can read my original post here:
http://www.peachparts.com/shopforum/diesel-discussion/316631-1985-300d-starting-issue.html