Quote Originally Posted by SurrealOne View Post
Today I ordered a 0-400psi sending unit and Dakota Digital air pressure gauge for in-cab use with my OBA project.


This underscores why I hate Sierra/Silverado wiring differences. I need 99-02 Silvy tails for my 04-06 Sierra due to wiring differences ... and yet the flasher module I need for my 04-06 Sierra apparently won't work in a 99-02 Silvy ... due to wiring differences. There's no consistency to it. How the heck are we supposed to know when things will and won't cross over between models ... especially when I call GMC customer service to find out which gauges will and won't work in clusters among various GM models and THEY can't tell me. LOL!
I understand your frustration. For future reference though (probably more for others than Surreal), all the wiring is the same except the harness from the junction box on the back of your truck to the tail lights. Both function the same, but the 2003-2007 Silverados had wires for one extra little marker bulb. For reasons I doubt anyone will ever understand, the Sierras didn't get that extra marker bulb because they didn't get the new tail lights. It wouldn't even be an issue if the extra hole on the 2003-2007 Silverado lights wouldn't let moisture in. That being said, its not TOO difficult to add the wiring for that marker light if need be.

I'm glad you were able to cancel that nak. I believe the main difference is that 99-02 had the EP29 as the main flasher and a separate flasher somewhere for the trailer lights, but the 03-07 had the LM487 (I believe the factory non-LED one is LM483 or something), and the newer style serves as one module that controls all of the signal and brake lights on the truck as well as on the trailer.