The computer's audio goes into the Aux input of yoour headunit.
Want to listen to the radio? Select it with the headunit.
Want to listen to a CD? Select it with the headunit.
Want to listen to the computer's audio? Select it with the headunit.
You're seriously over-thinking this...
And USB radio cards/dongles aren't designed to be used while going 60 down the freeway or dealing with buildings and such. Their reception is usually poor.
If you want PC radio, the best solution is the HQCT (Hi Quality Computer Tuner, I believe). Information here:
http://xtronic.be/?id=91&lang=2
I believe the mp3car.com store has or had them available for purchase.
If you're going to go that route, there is really no reason to retain the headunit. The PC's audio can go straight to the amp or amps. That's how my system is/will be.