think the most important part is figuring out which radio harness you have.
The under-seat factory subs I have seen all have a 14 pin harness connector. I believe 08 on have the 20 pin, so you may be able to just plug n play.
If you do have the 20 pin, I successfully adapted a 14 pin to a 20 pin for my brother's 09 forester using a 20 pin breakout harness from SVXdc which I soldered to the sub wiring.
Pretty much every bit of information you need on the factory harnesses are on SVXdc's website: http://ae64.com/
and pinouts for the factory sub here: http://www.iwsti.com/forums/gr-ice-s...questions.html