Having spent many hours on an in-car noise issue without being able to resolve it I'm hoping someone can point me toward a solution.
The issue I'm trying to solve is a buzzing noise in a car audio install present only while the normal lights are on and the brake pedal isn't being pressed. This eventually led me to measure on the rear lights. When the headlights turn on, the rear side lights receive a 100Hz PWM signal, ~25% duty cycle and battery level amplitude (so the bulbs share a common ground). When pressing the brake pedal this signal changes to 100% duty cycle and the buzzing noise is gone. In my unlearned opinion this either means this noise is picked up and amplified, or it causes some kind of noise to be generated inside the amplifier.
I 'checked' the ground point quality for the rear lights because every car audio/snakeoil expert seems to claim 'you have bad grounds'. Now I don't know how to check ground point quality or even what kind of measurements would be considered within tolerance, but alas. With the 25% duty cycle signal to the bulbs there's some 10mV and <0.2? between the chassis and bulb grounds. Does this imply the ground is 'good'? Or would I need to actually measure some disturbance in the supply lines to conclude this?
Back the the origin. This PWM signal is generated separately for the left- and right-hand sides of the car. Both are generated by the body control module all the way in the front of the vehicle. RCA cables with the audio signal run along the right side of the car. This led me to believe the buzz is picked up here, but it is probably not. I have cut the wires at the body control module, and rerouted them with new wire, far apart from the audio signal wiring. Hooked them up directly to the bulbs in the trunk but the buzz is absolutely identical - so I guess it's not picked up by the RCA cables and amplified after all.
I'm unsure how to proceed so any tips are helpful. Hopefully someone has a methodical or theoretically supported approach to this kind of issue, I'd love to learn! By now I've come across so much youtube advise including 'picofuse replacement', powercaps, grounding directly to the battery, noise level filters, ground loop isolators, using an amp with high-level inputs and more. For what it's worth: yes, the RCA grounds are fine. Ground loop filters hardly affect the buzzing. My RCA cables are not low quality, and have both a shield as well as drain wire. And I don't want to use high-level converters.