quick question

i would like to put an aftermarket amplifier in my town car, but keep my stock head unit, how do i do this? what i'm shooting for is to keep my stock speakers in the doors, stock radio in the dash, but change out the factory amplifier in the trunk for say a 600 watt amp to run subwoofers off of, and a smaller amp to run the door/rear deck speakers. is there a special harness i need to buy? will the new amplifier be compatible with the stock speakers/head unit ? cant seem to find any info on just switching out the amp, thanks, sincerely, jason