I'm currently using a Samsung Helix which is portable but I originally had a built in Sirius tuner in my leased Crossfire Roadster. It was an Audiovox unit and had a separate tuner box and FM modulator switch. I don't like the idea of installing equipment under seats where it can get snagged by moving parts or get wet, so I found a better location.

Like most Mercedes vehicles, the Crossfire has its engine control computer located under the passenger footwell. If you pull the carpet back you will discover a metal panel held on by three plastic lug nuts. The panel can be removed and there's a lot of space alongside the control module. Using this location means that you don't have any wiring going into the cabin, (except for the antenna). If you are going to put the antenna on the dash, it makes for a very simple install. You can easily fish wires from the radio area to this location and you can use the existing wiring to hang your wiring from to prevent any rattles.