When I bought my car last year, the previous owner had a similar Sirius setup in the car.
After a long time, yesterday i removed the dock, he didn't give me the module but just the dock. check out this center console
the dock is setup next to climate control. I liked the setup but had to remove because its uncomfortable to passengers.
This is the dock kit
Based on the dock spec, i had three wires in my car.
1) Power - This was tapped into big radio harness.
2) Antenna - Signal wire goes to the top of the car linked to a magnetic receiver.
3) FM Adapter (OP
this is the 2.5mm adapter you see in the center console) - i have this setup
on my car, this acts as a input switch, when the Sirius is turned on it outputs a 5v signal through the 2.5mm adapter, this turns off the vehicle fm signal for any interference and viseversa.
Now... I did some research this morning and you need fm modulator to make it work. because sirius had an inbuilt fm modulator, it can produce fm signal.
Something like this...