Don't take this wrong, but is the balance adjusted to one side, I have to ask that first. Did this occur all at once? The radio has seperate inputs to the amplifier for front and rear, right and left(total of four inputs). The amplifier has four outputs on 8 wires. The radio and amplifier share a common ground. I would speculate you have a component problem, either the radio or amp. If you had one speaker out, I would lean toward amplifier or circuitry. Since you have a whole side out, I would guess it is a radio issue.