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 Certified audio upgrades
OE-Look Enclosures for F30/F32/F22/etc
Add-A-Sub and Full-Bass Packages
Full Cabin 3W and 4W Upgrades
F10 Enclosure preorder- contact us for details!