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How to Identify the Wire Functions of Trailer Wiring on a 2002 GMC Sierra


2002 GMC 2500 HD left side box cable has 8 wires the question is light blue or dark blue the wire I need and what happens with the extra wire does it piggy back onto another pin. The one wire that is longer is dirty, it is actually light blue


Expert Reply:

I can help you determine the functions of the wires in the bundle you sent in a picture of by testing them with a circuit tester like part # PTW2993 but we don't have a way to identify them just based on color.

So have a buddy apply the various taillight functions of your truck while you probe all of the wire cores with the tester and you'll be able to know for sure the functions.

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expert reply by:
Jameson C

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Follow up Questions

Dave S.
8 wires in the harness 7 pin connector I need to find the answer for the 8th wire
Etrailer Expert
Reply from Chris R.

I was able to track down the owner's manual for your 2002 Sierra, in which it lists the functions of the 8 wire trailer harness you found. The light blue wire won't be used at all for the 7-Way connector. The other colors/functions are: Yellow - L turn/stop, Dark Green - R turn/stop, Brown - Tail/Running lights, White - Ground, Light Green - Backup lamp, Red - 12V charge, Dark Blue - Brake output.

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