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1500 Sierra: the wiring for electric trailer brakes..pickup

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I am trying to trace the wiring for electric trailer brakes on a 2003 GMC 1500 Sierra pickup, cheep model. It has trailer ready wiring, i.e. it has some wires run to the rear of the pickup for lighting. It also has the trailer brake moduel under the dash on the left side. My problem is the wiring from there to the rear of the pickup. I thought if I had the wiring diagram, I could resolve my problem.



HEllo Sheldon,


Although your's may not exhibit this condition, the technical bulletin may help.


Bulletin No.: 02-05-22-004C

Date: April 05, 2005


Trailer Brakes Applied When Headlights/Park Lamps Are On, Brake Controller Illumination (Modify Brake Controller Wiring Harness)


2002-2005 Cadillac Escalade, Escalade EXT
1999-2005 Chevrolet Silverado
2000-2005 Chevrolet Suburban, Tahoe
2002-2005 Chevrolet Avalanche
1999-2005 GMC Sierra
2000-2005 GMC Yukon, Yukon XL
2003-2005 HUMMER H2



This bulletin is being revised to add additional model years. Please discard Corporate Bulletin Number 02-05-22-004B (Section 05 - Brakes).


Some customers may comment that when the headlamps or park lamps are on, the brakes on the trailer are always applied, or that the back lighting for the trailer brake controller only illuminates when the brakes are applied.


The cause of this condition may be due to wiring changes within the vehicle electrical system for the 2003 model year.





Inspect the brake controller wiring harness jumper that is plugged into the vehicle relay block-body. The relay block-body is located under the left side of the instrument panel near the left kick panel, behind a plastic cover.

The 1999 though early 2003 wiring harness jumper that plugs into the relay block will have a reddish/brown 6-way connector and a tag with the last four digits of the wiring harness part number. Second design 2003-05 may have a white plastic connector with part number 5418 as well.

^The 1999 jumper is/was P/N 12171982 with a pink/purple tag identifier of 1982. This number has been superseded to 15366255, with a pink/purple tag identifier of 6255.


^The 2000 jumper harness is P/N 15366255. The pink/purple tag identifier is 6255.


^The 2001-2002 jumper is P/N 15086884. The orange tag identifier is 6884.


^The 2003-05 jumper is P/N 15085418. The orange tag identifier is 5418.





When transferring the brake controller with the wiring jumper attached from a 2001-2002 vehicle to a 2003-05 vehicle, a change to the jumper harness connector must be performed.




The information shown is for the 1999-2002 wiring harnesses and for the 2003-05 wiring harness.

To modify a 12171982, 15366255 or 15086884 harness for use in a 2003-05 vehicle, switch the wires in cavities "A" and "D".

To modify a 15085418 harness for use in a 1999-2002 vehicle (see model list at the front of this bulletin for applicable models), switch the wires in cavities "A" and "D".

Warranty Information

When the correct wiring harness P/N is matched to the specific model year of the vehicle, everything works properly.


I'll forward you a bit more shortly.




Here's more Sheldon,










I hope the information I have provided helps.


Take Care




Here's a bit more I found.


It appears that the dark blue for the trailer brake wire will be near the IP relay block and will be a blunt cut wire just hanging there.


Trailer Wiring Harness
Your vehicle is equipped with one of the following wiring harnesses for towing a trailer or hauling a slide-in camper.

Basic Trailer Wiring
All regular, extended cab crew cab pickups have a six-wire trailer towing harness.


The six-wire harness is located at the rear of the vehicle and is tied to the vehicle's frame. The harness requires the installation of a trailer-wiring harness connector, which is available through your dealer.

The six-wire harness contains the following trailer circuits:

  • Yellow: Left Stop/Turn Signal
  • Dark Green: Right Stop/Turn Signal
  • Brown: Taillamps
  • White: Ground
  • Light Green: Back-up Lamps
  • Light Blue: Center High-Mounted Stoplamp (CHMSL)

If you need to upgrade your trailer wiring harness, an additional 2-wire harness and 7 pin universal trailer connector are available from your dealer.


Heavy-Duty Trailer Wiring Harness Package


Your pickup may be equipped with the eight-wire trailer towing harness. This harness with a seven-pin universal trailer connector is attached to a bracket on the hitch platform.*

* The Center High-Mounted Stoplamp (CHMSL) wire is tied next to the trailer wiring harness for use with a trailer, slide-in camper or cap.

The eight-wire harness contains the following trailer circuits:

  • Yellow: Left Stop/Turn Signal
  • Dark Green: Right Stop/Turn Signal
  • Brown: Taillamps
  • White: Ground
  • Light Green: Back-up Lamps
  • Light Blue: CHMSL
  • Red: Battery Feed
  • Dark Blue: Trailer Brake

If your trailer is equipped with electric brakes, you can get a jumper harness (electric trailer brake control) with a trailer battery feed fuse from your dealer.

If you need to tow a light-duty trailer with a standard four-way round pin connector you can also get an adapter from your dealer.



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