Hi Tim, Your car should have a small fuse box located in the engine compartment along the passenger side fender near the radiator over flow tank. I think the box has two fuses at least one of which is specifically for the radio. Most power antenna that I have seen have a ground and two power inputs. One input is always +12V and the other is switched power from the radio. I think the second fuse in this box could be used for the antenna.
Jeff C.
1970 280SL 4-speed