there could be something loose on the board inside the avic,
bad preout? is the amp that your using driving all of the speakers (no a sub amp) sorry i dont know polks model numbers, if so i would switch the rca's (at the headunit) from right to left (the left outputs are plugged into the right RCA cables and the right out puts are plugged into the left RCA cables) and see what happens, if the problem switches sides then the problem is with the avic (either with the RCA connectors them selves or something inside the unit).
if the problem doesn't switch sides (you are still having problems with the right side) then the problem is not with the radio so continue trouble shooting
if problem still persist in the right side (after the switch) then switch the rca's at the amp this time (the right cable is plugged in to the left input...) now your balance is correct, and fading to the right on the radio fades to the right on the speakers.
now if the problem is happening to the left side of the car, then the problem is with the RCA cables themselves if the problem is still on the right side then the problem is your amp, wiring, or speakers (since you have thoroughly tested all of the wires then that leads me to believe that the amp is your problem)