Yea... big spongy gasket. The old one was no thicker than wet suit neoprene.
The install would have been easier if I had a longer flathead screwdriver I could have modified for pushing the clips through, and if I'd known that pushing those clips through was only going to net me about 2 inches of slack.
Ultimately, I opened the rear slider, and slid my hand up into the headliner to hold the plug in place while I swapped harnesses. If I had to do it again, with the custom flathead, I could do it all in probably 5 minutes.