I been searching some Info for that problem because my G6's brake lights and rear blinkers haven't work at all so what I found is that the BCM(body control module) has corrosion on the connectors specially on the C2, if you don't know where the module is at, is placed on the right side of the front panel below the air bag and on the side on the stereo. There you will have to clean the C2 connector pins with a nylon brush and some cleaning gel or DIELECTRIC grease as the GM 12377900. Is about $15. And you can also unlatch the C2 connector the disconnect, then latch it again and connect. Repeat this steps about 3 or 4 times so you can clean the corrosion away. Hope this info will help you.