You just need to buy a unit that goes through an FM tuner. You have to find a frequency with no interference for it sound sound perfect. Sirius used to make their units start at 87.9 which is interference free but due to some dumb FCC regulation nothing can transmit on that station since it's an Emergency station.

Sucks for me in my work truck cause I travel in a "tweener" area between 2 markets so I have to switch my station a few times for it to be clear.

Get the Sportster Replay, it has a button that you can preset like 5 different FM stations that you can switch to on-the-fly.