^ What DB said.
If you get a voltage, it must be the motor (or the teeth inside) that's knackered. If you don't it must be either the aerial relay, or the that the HU isn't sending the UP signal to the relay in the first place. All relatively straightforward to test.
When I plugged my new headunit in, it wouldn't operate the aerial. It turns out that whatever wiring harness had been fitted, it wasn't passing on the aerial signal as required. Autoleads harnesses are the ones to get. There's 2 terminals on the back of the aerial relay that directly control the aerial. When one goes +v, the aerial goes up, when the other goes -v, it goes down. I can dig out the info if you still need it.
Steve