sound through brake

Find out if the noise only occurs with AM or FM. Try other accessories in the car to see if they too make noise. Use any swtich that passes a good source of current (wipers, horn, signals). If it does, I would suspect a fault in the antenna. Check that the antenna lead adaptor is the correct one and allows a good ground for the antenna to radio. If the noise is present on all sources, you can use capacitors across the brake switch to eliminate the noise. Three small polarized caps should do the job. One across the hot and switched wires, one from the hot side to ground and one from switched to ground. This should take car of it. The brake switch could also be replaced. You are likely hearing the arcing of the switch contacts. After a few years the contacts build up a lot of carbon and create noise.sparky

The customer finally showed up and looked at the ground on the battery and it was extremely loose.  So I got that nice and tight and now the pop can only be heard through the rear speakers only if the radio is all the way down and you don't rev the engine.  It has to be complety quite to hear it.

So the popping only happens when the engine is reving and the brake pedal is pushed in?