Installing amp with a factory amp

I bought a 2005 Pontiac Vibe, with a factory amp under the passenger seat that powers the regular speakers and a built in Monsoon speaker. I replaced the head unit, and installed my Kenwood Excelon KAC-X521 and 2 12" Dual's. I have everything set right, but very little sound is coming from the subs, is there special wiring needed to use an aftermarket amp with a factory amp?