Car Stereos: high pitch radio squeal when using brakes, high pitch, pain in the butt

Question
I have an '03 Envoy that has developed a high pitch squeal on AM radio that drops in pitch when applying the brakes. If the brakes are continually pressed, the squeal drops into a low frequency garble and the nothing can be understood.  After a short period the pitch begins to rise until it is back to the beginning frequency and then repeats.  This only happens on some AM radio stations and most prevalent on the lower frequencies.  It also only occurs when the car is warmed up.

Answer
Hi Dave,

This is a tough issue to deal with, since it means you're obviously getting some nasty interference somewhere, and it's only in a specific frequency.

The bad news, is it's going to be a pain in the butt to diagnose.

A good first step might be to see where the lines for the brake lights (what's probably causing it) are run, and finding the speaker wires that are run in its vicinity (likely parallel) and increase their shielding or replace them with new wires.

Justin