run the speakers you have off the head unit and power the woofer with the amp. also you could use a pair of larger speakers on the back (6x9's are usually used) and some "tune up" tweeters up front to balance up the sound stage (try using the speakers you already have in the front doors with this "upgraded" setup).
pay good attention to the + & - on the wires otherwise you won't get the right sound as (in some cases) the speakers will cancel themselves out (specilaly the bass)