Intermittent sound from the radio

The radio, which has the built in disc changer and Sirius XM receiver all in one, will intermittently cut off the sound. Sounds like the station is going in and out of service. Not the case. The problem lies in the instrument cluster dimmer rheostat. If it is turned on the brightest setting, not the "on" position, the radio will not play. Power is on, but no sound. If you slowly turn the dimmer down, the sound comes back on. I have to drive at night with the instrument cluster lights dimmed way down for the radio to play. I have checked a few grounds on the frame but to no avail. I have not pulled the radio out yet and searched throughout the dash. I did replace the headlight switch assembly, but that did not work either. Help!