You can remove the head unit to do everything you need to do (it's MUCH easier than removing the rear panels to get to the speakers). You can either wire your sub amp to the rear speaker outputs on the back of the head unit through each speaker wire and run the lines back to wherever you choose to mount the amp, or, you can wire in a line converter (
http://compare.ebay.com/like/3305366...Types&var=sbar) to run RCA cables to the amp.
The power antenna wire (usually blue) is in the harness behind the radio as well.