Thanks for the question.
First, disconnect the taillamp and connect a test lamp to the light blue wire. Have someone hit the brakes. The lamp should come on.
If not, look under the bed of the truck, near the rear bumper. There should be a junction block there with a few connectors going into. Unplug the connectors and check for signs of melting or corrosion. Test the light blue wire there too for power to the brake lamps when the brakes are pressed. If the left side stop lamp is working, then there is either an open in the brake lamp feed wire or the junction block has an open circuit in it since the brake lamp feed wire splits inside the junction block to go out to each lamp.
I checked the blue wire while the brakes were applied, Nothing. I even checked the left side just to be sure my test light was working. Also I checked the connectors at the junction blocks under the truck like you suggested, but they were OK, not even dirty. I guess I'll be taking the truck to the local dealership to get it fixed. Thanks anyway.
Ok, the wire splits and goes to each lamp INSIDE the junction block. If you have power to one lamp and not the other, then the issues will be inside that junction block.
I have included a picture to help you see what I am talking about.
So replacing the junction block should take care of it. This is a repair that you can do yourself.
Thanks for the wiring diagram. I will replace the junction block.
Thank you for the help.
Let me know if I can help further.