Need circuit for antenna up from Sirius radio

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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