Unless the battery cables are badly corroded, i'd say the starter quit. Sometimes cables can make just enough contact to do this- they get too corroded to pass a lot of current. If you jump the solenoid wire on the starter to the big battery lead with a screwdriver or something, see if it turns over. IF it does, then you've got some kind of wiring problem in the car. If it doesn't then it's new starter time. I'd recommend getting a used one or getting yours rebuilt- new ones are far too expensive.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE: 625K total