Here's a photo of what's behind my head unit...
Since the photo posted upside down for some reason...
The big silver plug on black wire in the top right of the photo is, I believe, the antenna's signal --- that one plugs into the back of the head unit in an obvious position, no problem.
The solid blue wire with the blue splice at the end, in the center of the photo, should be the antenna's power. That blue wire comes out of the black plug that is connected to the truck's radio harness, but does not connect to anything on the head unit itself --- nor is there a spare wire or an obvious location on the head unit that could be used.
The silver plug on black wire in the bottom left of the photo is the "baby coax" I mentioned, it's definitely smaller in diameter than the bigger one in the top right. This baby coax is not connected to anything, nor is there an obvious location for it in the head unit.
I'm thinking that blue wire needs to connect to the radio somehow so that it knows to drive the antenna up and down. What can I splice in on to?
And the baby coax... Maybe that's the factory amplifier or something...?