line out converter

Here are some sources:  http://www.peripheral-aamp.com/peri_faq_main.htm      http://www.davidnavone.com/

If you are only adding an amp for subs, you can use a two-channel converter.  It picks up the signal from the OEM speaker wires and converts to RCA's for amp input.  Easiest is to tap rear speaker wires because of their location, but you may want to tap the front ones instead so that when you fade the deck to attenuate the rear speakers the sub input won't fade out with them.  Buy a high quality adjustable LOC and you will be able to adjust it for highest signal-to-noise ratio and lessen the chance of adversely affecting the quality of the sub input.  The P35 is a good one for you.