Ok, got it. You have to slide the assembly down enough to allow the top to clear the dash and it comes right out. It takes a little prying/force, applied in the right way of course.
Anyway, I installed the new antenna switch and the antenna still won't work: with the key off and the switch off, the antenna motor runs as soon as the harness is plugged up, until the mast is fully retracted, and power stays on until the antenna unit starts to heat up. It will not respond to the switch at all. It's exactly as if the antenna were getting 12V to the Retract terminal (whichever one that is) all the time.
I'm wondering if I could have swapped two wires when I removed this trim piece to change out the ACC unit; when I removed the harness from the rear of the plug this time, I noticed one of the leads came loose from the harness plug and stayed in the switch, and I had to remove and install it. If two of them came loose before I could have swapped them, who knows? The antenna motor did not run at that time anyway, so I did not see any additional problems.
If anyone can offer advice, I will probably listen closely.