04 Vibe w/ moon tunes Bass in Front but not in Rear speakers

[mkarps]

I'm not sure where to go first. Dont really want to replace the HU, or Amp, or speakers without a little advice from the Pro's(you guys) first.I have bass in the front speakers but when I fade to rear it has no bass and very tinny. I understand the amp powers all four door speakers so assuming it is not the amp? Could it be the head unit? I have the common display issue with it so should I try replacing it and hope that is the speaker problem? I can pick up a cheap one on ebay to pop in. Would I need a code though to make it work? Any help would be great!