Looks like Q24 on the CPU board activates a relay that drives the motor for the tombstone. Q24 leads to connector CN12 pin 8 of the CPU board, brown-violet wire. From there, I would follow the wire to the relay...I would imagine the relay is probably on a little board under the playfield somewhere, but I'm not sure. Check page 33 of the manual for this chart:

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Cord missing the ground prong is probably not related, but I would definitely fix that anyway.

Looks like the relay will feed 20 (28? print is a little hard to read) volts AC to the tombstone motor. Did you check for AC volts at the motor when you were testing earlier?

Agree with @Boat, corrosion sounds like a very possible culprit...in fact I think I see some of that blue/green battery leakage/corrosion on the connector you posted. That would probably explain why it's so difficult to get the connector to pull off...(there's no trick for pulling that connector, it should simply pull off like any other connector on the board)
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