I found this on http://bcae1.com, I don't know if this is what you're looking for, but it kind of sounds like the problem you're having:
This circuit was designed to invert an audio signal. Most car audio amplifiers have one 'normal' channel and one 'inverted' channel. The inverted channel is needed so the amplifier can be bridged. Some of the older Orion amplifiers did not have an inverted channel and therefore needed a 'bridging module' to bridge the amplifier. To use this circuit, you would connect the left signal directly to the amplifier and the right channel to the input of this circuit. The output of this circuit would be connected to the right channel of the amplifier. You would then use the 2 positive speaker output wires for bridging. The left channel will be the positive output. This circuit will probably need to be powered by a constant source to prevent a turn on pop. If you don't want to connect it to a constant power source, connect it to a source that's controlled by the ignition switch. Do not connect it to the remote out of the radio.