So what you're saying is, when the brake pedal is pressed OR when the C/C applies the brakes, and various things happen (most notably the brake lights come on), it affects your left channel?
I would start swapping L and R channels at various places in the signal chain and see when it moves and when it doesn't. I suspect you will find it to be some induced noise associated with the brake lights.
musicar northwest portland or musicarnw.com
Web site migration complete, still repairing problems. Sorry!