My new volvo nav system came with satellite radio prep.  What do I
need to buy in order to enable sirius radio?

You can visit www.sirius.com and there should be a list of radios you
can purchase from that site for easy hook up to your vehicle.  After
that, it's a simple installation, followed by a quick call to Sirius
(The number is provided on your package) with the radio serial number
and you'll be ready to go with their radio system.