well i just installed my two Rockford Fosgate SUBS (P3DVC) And My RF P3002 Amp. I did not want to change my deck so i went with the In line Convertor 17.77 at WAL-MART....So first i must commend the Volvo engineers for putting the battery in the trunk and for running a light blue stripped power antenna wire up the left speaker connections so i did not have to remove the head unit.

(REMOVE THE NEGATIVE BATTERY LEAD)

1. Splice into left and right rear speaker Gray is negative gray/white is pos. on th eleft side tap the light blue /wht stripe wire you will need this for the remote amp turn on.
2. Run your power from the battery use some good quality wires.
Then Run your ground i used the seat bolts an excellent ground.
3. Wire the amp and connect everything and done.

I think in the future i will add a deck because i am sure the sub would sound better with a dedicated RCA from a good deck but i am being cheap for now.

i hope this helps.

Quite possibly the BEST thing you can do is to replace the deck, when running subs and upgrading the sound of the system, a new head unit provides more power/speaker as well as a voltage boost to the amp via RCA's. This means that there is more current running through the RCA's to the amp which will give a stronger, cleaner sound

And that is what i did the rca in line was not enough. i purchased a Kenwood 791 and oh th esound is way much better. I love it.

I have a 2005 S80 that I would like to replace the OEM radio with an after market system. It appears as if you have done this. Did you loose you steering wheel controls? I have been doing some online research and have not found any steering wheel control adapters for the S80 but I have found adapters for the V70 which as far as I can tell in this matter is the same as the S80. Just trying to find somebody who has already removed the OEM radio with aftermarket and what issues they have had.

JPMan, you want to order a Dietz adapter to retain steering wheel controls.
http://www.dietz.biz