infinity sound adding a subwoofer/amp

Just wondering on the best way to add an amplifier to my factory Infinity sound system. I like the sound the factory system produces, but have found a custom box for subwoofers for cheap for my 2002 QuadCab. Any help would be appreciated. Thanks!

i just added a deck in my 05 ram and a thunderform enclosure in my truck and the wiring harned was hugew for the deck to bypoass the factory infinity amp which is buried back in the dash behid the glove box