No Sound Rear Speakers Help

[AutoXBot]

I've recently bought a 4 channel amp (Hifonics HFI 55) and tried hooking my Polk Audio db651s speakers to it. Front left/right speakers emit sound okay, but the rear left/right speakers only have a low sounding base coming from them.

My head unit is a Pioneer Avic U310BT. The amp kit I'm using is Monster cable. I have speaker wires running from each speaker to the amp directly. Two RCA cables (one set of two for the front and back, total of four) running from amp to the back of the head unit.

What am I doing wrong? Should I disconnect the speaker wires from the head unit's wiring harness, since they're not connected to the speakers anymore?

I'm also planning on adding a amplified Bazooka tube. I was going to wire this to the rear speaker wires from the head unit, since all four RCA inputs are taken by the amp. I have yet to do this, 'cause I'm trying to get all four speakers to work first.

Please help.