That's probably the "RAQ". It should be marked just below the tuner knob. If so, that's the same one I have. Isimple makes a nice little interface. Got mine on Amazon for $80. It allows you to play/charge an Ipod. No display on the radio, but you can skip/ff/rw songs with the steering wheel controls and also change playlists. Plugs into the 10 pin port on the back of the stereo. (if you have a rear dvd it needs to be disconnected). The unit also has a second input via RCA cables that another device can be connected to.
It shouldn't affect your Sirius. To access it you simply press the "Mode" button to scroll through the multiple inputs.