So when you probe both wires on the the connector (BK/Y & B on a '93) you get 12v with AC on, but when you plug the connector in, the clutch doesn't engage?
Reason I ask is if you're only probing the +12v wire and the other probe is grounded to case or bracket, then you may have a bad ground wire going to the connector. I've done this before and stumped myself!
If you're probing both wires on the connector, other possibility is there's a loose wire/connection inside the connector that's not making contact when the connector is plugged in, but when you stick that big fat probe into the connector slot it makes the connection. Had this happen before too - connectors do wear out and are easy to replace.