Ditto on check engine ground. Once when I had the cluster pulled out in my 1985 300D, I actuated the starter (forgot why) and noticed the speedometer cable smoking. That clued me to sand the connections on the ground strap from transmission to body, then coat w/ silicone grease. There is also a ground strap from BATT- to the frame. If it is a voltage loss issue, you could find where using a multimeter while trying to crank. All the starter cares about is the voltage from its case to the big stud +12 V feed. If >8 V while cranking, a good starter should work. You could have excessive drop on either side. The solenoid is integral to the starter (unlike 60's Ford), though I think can be unbolted.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans