Line Input to Sub & Amp

I received my Prius with the base sound system two weeks ago and purchased a powered Infinity Basslink 10" Subwoofer:

http://www.infinitysystems.com/infinity_ca..._subwoofers.htm

The battery power and ground are no brainers to hook up (although I would love to see how other's have hooked up theirs) and the auto turn-on is based on the signal that will come from the line inputs so I don't need to run a wire from the rear of the head unit.
My question is: where is the best place to splice into the rear door speakers so that I can send the signal to the amp that will be mounted in the cargo area? I have not been able to find any info on where the wires are located, how to access them and how to best run them to the back. Any and all comments are welcome. Thanks in advance!

 

Thanks Dan Man, but I really meant, where would I splice into the wires that lead to the speakers in the door. I understand that the wires are leading out from the door and to the deck, I would just like an opinion on the best place to splice alogn that route (most convenient) and how to run it to the back...
Thanks again...

 

No blipping and popping - just good sound. I have it installed in the storage hatch cover. Doesn't seem to be an issue and I can still take off the cover if needed, albeit carefully. Carpet doesn't fit anymore though...oh well...