Did they work before you changed the head unit over? If so then it's clearly down to you connecting the iso block connectors on the rear of the unit. If they didn't work initially then id start by popping off the speaker covers on the rear doors and checking the wiring on them, make sure to have a good inspection of the wiring that passes through the corrugated rubber grommet that runs from each door to the door pillar as breakages can occur in these with age.
