Need circuit for antenna up from Sirius radio

I need a circuit that knows when the power is on at my Sirius Stratus radio to tell the antenna to go or stay up. Problem- my current auto radio has an aux input which I can use for my satelite radio. Great direct connection. The problem is that when I set the auto radio for aux the antenna goes down. Is there a way to sense power on the 5V input to the satilite radio and then tell the antenna to go up. The auto radio sends a 200ma signal to the antenna which closes a relay and sends the antenna up. I was hoping not to have to go inside the Sirius radio case to pu maybe the display light on to send to the antenna. Any one have a good idea? I'm already building a regulated 12v to 5v power supply that will go behind the dash so I don't have to use a cigarette lighter plug in for power. Aprpreciate any advice. Gary