why would you need to write software on the PC? I figured once the interface can send stuf to the pC, then girder could sort out the rest?
you didn't see any point in controlling the pc with the headunit? for one it's a great input device which looks good and feels good. It;s a great thing to have alongside a touchscreen, if not replacing the touch screen completely. Then there's the headunit display. If you go as far as sending the song title back to the headunit then you will have mroe space on the pc's monitor for everything else like sat nav etc.
This way, the joystick (either the IR one or the 3.5mm plug one, whichever fits your radio) could again be used to control the PC, but at the same time can be used for the headunit when the source isn't the cd changer.
Otherwise I'd want two joysticks, one for hte PC and one for the headunit which wil look very unprofessional.