where are the mid components mounted? in the door next to your feet? if so, i can see why the rears sound louder. the sound in the mids get trapped in the leg area of the front seats, while the rears get to bounce off the rear windshield and fill up the rest of the back. and your tweeters if mounted earheight, fills the upper half of the front.

or it might be that the fronts are out of phase and are cancelling each other out. i'd check on that.