if you just want to add subs, that's not a problem. You can either use a line out converter (loc) to tap into the rear speaker wires, or you can get an amplifier with high level inputs. This will give you the input signal, and the amplifier and subs would be set up as usual. The easiest option is to get an amplified subwoofer system from Bazooka, Infinity, Alpine, etc... They include everything in one chassis, and can add enough bass to round out your factory system. If you want a lot of bass, then look to a separate amplifier and sub(s) set up.

this is an loc:

http://shopping.canoe.ca/shop/product--catId_1001077__locale_en__productId_1850456.html

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2004 300M SPECIAL
Alpine CDA 9833
Alpine Pxa-h701
Alpine Rux-c701
Rockford fosgate p400-4
Rockford fosgate p4004
Hifonics BX 1205D
JBL c608 GTi
JBL p693
Infinity 2.5" center channel
Alpine type R 12"(2)
all active SQL set up with 1900 watts rms