Yup, I use the Head Unit of the car so I leave the PC volume to about 50% and then use the SW to change the HU volume - You can edit ibusrr.ini to change the pc volume.
To use the IbusRR plugin you need this:
http://www.reslers.de/IBUS/.
You can connect the [12v / Ground / ibus] cables to:
1. The boot, from the cd changer cable:
http://indashpc.org/vbullettin/viewtopic.php?=&p=2776
2. Back of HU:
http://indashpc.org/vbullettin/viewtopic.php?p=1373