[BNaylor]

Ok. There is a RF filter which is used for the rear defogger grid and radio antenna. When it goes bad it shorts out and may pass 12 volts back to the radio via the antenna coaxial cable. The RF filter is located at the rear passenger side C pillar and package tray where the speakers are located.

Your symptom is different but I believe it is a similar problem. Normally what occurs is the radio works fine until the defogger button is pressed.

To help isolate and troubleshoot the problem, remove the defogger mini-relay #9 in the engine compartment fuse box and then see what the radio does.