I suspect a wire has shorted or perhaps it's something very simple. Has anyone replaced a brake light bulb recently look to see all brake light bulbs match and are the correct bulb for the car, if a single filament bulb was put in the wrong socket or a double filament bulb put in a single filament bulb socket feed back would cause the problem you describe.
We had it in for an oil change at the Chevy dealer. It was given a very cursory look...there was one side brake light out which they changed...but they weren't really listening to my wife...so the basic problem was not dealt with. We'll pursue your suggestions. Thank you very much.