Is it making ANY noise when the key is turned (clicking, etc.)? As far as I know, the starter can only be accessed from below the car (someone please corect me if I am wrong). How DIY are you? You first need to get to the wiring on the actual starter with the multimeter, and make sure that you are getting voltage on the high voltage side (with the key turned to start, of course). Try that and report back. By the way, question #2 is probably irrelivant, for you're not even cranking. If it was cranking, just not starting, THEN I would worry about the pre-glow system.

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1989 300E 144K