This video explains the audio wiring on a my Volvo S60, this should apply to S60 from 2005 to 2009 V70 from 2005 to 2007 and maybe xc70. The original head unit was a HU650 but this guide is still useful for owners of other model stereo's as i explain some of the differences between my setup and others.

I also explain what other wires are for in the green connector. The centre speaker runs through the power connector (green) usually its powered by an internal amp, dedicated just for the centre speaker however on the HU650 it has an internal amp but not for the centre speaker.

i have wired the centre dash speaker to an amp and im using it as a subwoofer. How long it will last im not sure but if it does happen to blow i can always replace it with a 8 inch subwoofer. My amplifier supports 4 and 8 ohms so its fine to run the dash speaker without issues.

In my car the door speakers and the shelf speakers are 8ohms, but wired in parallel so they will be seen as 4 ohms at the head unit so there is no issues changing your head unit to an after market one. You wont need to change the speakers also unless you really want to that is.