Hello Mr. Shiftright. Thanks for your input, I had tried the front bulb as well with no luck. Also, changing that Multi-function switch didn't do anything either. What DID work was actually changing the Tail Lamp Circuit Board. I got one online for about $20. When I ordered it I thought about getting the Left one as well, but didn't since I didn't know for sure if that was the cure. It was, after replacing the actual circuit board that the lamps plug in to, everything worked like a charm. I should have ordered the Left side one since about 3 months later, the Left rear developed the same trouble -- got the new Left side Tail Lamp Circuit Board and all is well again. So, any Yukon/GMC owners -- if you ever develop a problem with your rear brake/flashing lights and it isn't the Bulbs, get the new Circuit board (buy both because if one goes, the other is sure to follow) and you will be back in business. Thanks again, shiftright, for your response.