https://www.gmupfitter.com/ will have any information you could possibly want. navigating their website I have found to be a weird layout.
Most of the time I get lucky by typing out in google gmupfitter wiring diagram then type the make model and year of the truck
But just going to their home page I end up not finding much.
Others here may have a better ways to find data on that site. But trust me anything you could possibly want is there.

 

Wow! Thanks, that is perfect. Page 90+ shows the pinout perfectly, great reference, thanks a lot.

 

Erich, I am having same type of issues with my 2003 GMC Sierra 2500 HD, left turn signals hyper flash and left rear turn signal bulb does not work at all. Bought new rear lenses with wiring harness from Rock Auto, replaced entire left rear assembly with new harness going to connection box. Replaced under dash turn signal flasher unit and replaced junction box. Still same, no turn signal light in left rear, left hyperflashes, front turn signal comes on but hyperflashes as well. I checked ground on left rear turn signal socket and got continuity to bumper so good ground. Looks like not getting power signal. Do you have the pinout diagram for the junction box ? I looked at GMOutfitter but could not find pinout yet for junction box and blue plug. Likely have issue with left turn signal power wire from blue plug, need to track down towards fusebox to see where I am losing signal. Any information you can share would be greatly appreciated! Bill Earls, Roscoe, IL Thank you.

 

Wow! Thanks, that is perfect. Page 90+ shows the pinout perfectly, great reference, thanks a lot.

No problem glad I could help. Please post the search that you used to find what you were looking for to make it easier on others that may have a similar problem plus also suck like me at navigating that website.

 

So haven't found the fix yet, but here is what is interesting:
Using the website provided above, the blue connector on the junction board by the spare tire is what feeds everything in the tail lights. Running a ground from the negative battery terminal to the negative side of a test light probe, I checked all the functions (turn signals, brakes,etc) and they all worked as the should at least when probing to the blue connector. The ground however on the blue connector was not showing conductivity. I think (and correct me if I am wrong) there is a grounding point on the frame just back from the driver side front tire. It looks like the ground for the blue connector grounds there. There was no conductivity between that ground wire (even if removed ) and between the ground pin on the blue connector. Tracing the wire bundle back, I happened to squeeze the flexible wire protection and found it was full of water. I wonder if there is a junction in there for the ground that has rusted away.

 

Sure sounds like you are on the right path.

 

So it is definitely the ground is open on the blue connector at the junction box. I have factory trailer wiring so I jumped the trailer ground the the blue connector and everything works normal now. Just need to find where the break in the ground is.
Thanks for all your help.

 

So if when you put the turn signals on and the rear tail lights look like a fire engine/disco, Pull the blue plug on the junction board near the spare tire and check the lower left pin (black) for continuity to ground.

 

I am glad you got it figured out. That GMupfitters website is great.

 

@bearls99 FYI, that member has not been on this site for over 4 years. It would be better for you to start your own thread.