You could go with an adapter if you want to, but its not necessary. If you want to do it cheaply, just buy a cheap set of RCA wires, cut off the female end, and splice the male end into the rear speaker wires.
Also, our cars have a speaker saver function that turns the bass down when you turn the volume up. Unfortunately there's no way around that. It's annoying, but if you adjust the volume on your woofer's amp accordingly you should be fine.
Here's a link to the wire diagrams:
zerothread/157919