rear speakers low power factory amp? Help! Options

[joy_zze]

I have a
1999 Celica with 4 6.5" speakers and the little
tweeter in the door. I am pretty sure there is only
one factory amp in the console. For some mysterious
reason, the rear speakers are only getting about 1/4
the power of the front speakers, and very little bass.
I replaced all 6.5 inch speakers with JL audio, and
replaced the head unit with a pioneer mp3 player.
Still, the problem persists. I have double checked the
head unit installation, and all seems to be fine. Is
it possible the factory amp only partially crapped out
on the rear speakers? If so, what should i replace it
with? I don't need anything fancy. The front 4
speakers sound great. I'd just like the rear speakers
to sound as good. Should I (could I) just bypass it? Any help/hints you could offer would
be greatly appreciated, as I am getting quite
frustrated! Thank you in advance, Joy

[joy_zze]

BTW, I appreciate the help of both of you. But it can't hurt to try it first without an amp right? . . Before I crank out the bucks. I will keep an ear out for the distortion you mentioned Shid, and definately get an amp if it sounds bad. What would you recommend (high end - low end)?