I have a 96 blazer that I use for a hoopty, I am having an electrical problem. I was wondering if anyone could help. I had the alternator tested and it was good, had the battery tested and it was good, and I had the starter tested and that was good. When I am driving the volts on the gauge go from about 13 to 9. If I turn the rear defroster on it goes almost all the was down to 4. What could be wrong?