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

Updated 08/25/2025 | Published 03/11/2020

Question:

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

asked by: Dave S

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.

expert reply by: Jameson C

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Dave S.

3/18/2020

8 wires in the harness 7 pin connector I need to find the answer for the 8th wire

Chris R.

3/19/2020

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.