After sliding the red locking tab, there was just no way I could get enough hand up there to both depress the release lever on the connector, and get a firm grip on it to pull it apart at the same time.
I fished a long tie-wrap up there and around the wire harness, then fastened it so I had a loop I could pull on. Then, while using the stretched out fingers on the one hand I could fit up in there to push on the release button on the connector, I used the other hand to pull on the tie-wrap loop and pull the connector loose. I made sure I transferred the tie-wrap to the TST harness for future removal of it. I wouldn't pull a connector apart by pulling on the wires (great way to pull them right out of the connector) if there was any other alternative, but I couldn't find one for this job.