Sterio amp???

i think, dont quote me on this you could use a normal car audio ampliefier and run them throgh the high pass filter on the amp, but you may find the rear speakers are amped through the standard amp in the boot along with the sub,

best bet if you want good sound quality is to rip all the standard stuff out and upgrade all of the speakers etc, as the speakers them selves are one of the main reasons, along with the low power outputs from the standard amps

Just adding an amp will not produce any better quality from the stock speakers, just louder distortion and eventual self destruction. Add a proper sub woofer to get better quality and far better sound.