A couple of options:
1. Get an HD radio and XM package for the PC and throw the headunit away.
2. It might be possible to hack the aux input button on your HU, if it has one, so the PC auto switches to the PC sound for the GPS instructions.
3. Does the car use an amplifier or is it all done in the HU? If it uses an amp, Mitchjs makes a USB audio switcher that I think can be controlled by Ride Runner. You'd need to figure out the output lines of the car's HU to the amp and tap/connect them to the switcher, then connect the switcher to the amp.