While I do not know if it will play mp3, I can comment on speaker setup. The front door speakers are 3 1/2" woofer w/ a 1" soft dome. Rear door are standard Ford 6x8 full range. I replaced all with Polk coaxial speakers on sale at crutchfield (29. bucks a pair)- net cost w/ postage and wiring harness adapters was about 70 bucks. Greatly improved sound. Good instructions for install. Took me about one hour to install all four. Interestingly, the door panels on the left side of the car did not have the battin sound insulation- perhaps XLT door panels? in any case, I'm going to bring it to the attention of my dealer and insist that the panels be replaced, or the sound insulation be added. Next up- replace the sub-woofer. Again- good instrucions from Crutchfield. Oh yeah- another really GREAT option which is available for your head unit is the Mobile-Ease Blue tooth adapter- works GREAT- transforms your bluetooth cell phone to a hands free through the stereo. Not bad- certainly way ahead of GM.