Never mind guys, I've got it!
I got the wiring diagram - the relay is supplied via the pushbutton control unit, and then the voltage goes through the ETR switch to the relay. It ends up being that the contacts inside of the ETR switch were a bit dirty. This explains why there was voltage present at the relay socket, yet because of the dirty contacts there was too much resistance to allow enough current to pass and actually trigger the relay.
I actually disassembled the ETR switch and cleaned the contacts, and now it works fine, except for the fact that it is probably now out of calibration. I will actually get the new switch, but for now this at least allowed me to track down the problem and verify the cause.
Thanks again to everybody!