A 20-pin T-harness for Subaru's under-seat subs won't work -- the Kicker sub uses a different pin-out for the 8-pin plug (in particular, the power and ground leads are on different pins). Plus, the harnesses for under-seat subs are way too short.
The OEM harness for the Kicker sub has high-pass filters for all of the speaker lines going out to the door speakers. Do you still have any of that wiring, or did your friend not give you any part of the original harness?
Read through this thread:
New 2011 11" Trunk Mounted Subwoofer
The best way to go would be to see if Subaru sells the Kicker harness as a replacement part. Check the many online Subaru parts vendors for better prices than your local dealer (or even ask your dealer if they'll match an online price with shipping -- some will).
If you want to roll your own harness, see my homepage (click on my username). Harnesses from anywhere else will be missing several crucial wires you need for your OEM radio (dimmer, SWCs, CAN bus).
BTW, have you looked inside the Kicker's amp module?