I'm posting this for a friend of mine. He has an 01 s60 and wants to add a sub to the stock stereo. I wanted to know if anyone has done this with theirs and how you did it. Any help would be much appreciated.

Run the adapter from speaker level to RCA off one of the rear speakers and mount the AMP in the trunk.
The battery is in the trunk already.

Also check to see if JL or MTX have a vehicle specific enclosure for the car. They take up minimal amounts of space and are pretty much hidden.

*Edit* After checking the JL and MTX sites, neiither of them have an enclosure for any Volvos :-(