My recommendation is to get the correct harness. That way, if anything should go wrong, you have some legal recourse.
Several members have mentioned that you can modify a 2002 harness to work properly in a 2003. But if you do that, you have modified the manuafacturer's equipment, and you assume the liability if there is a problem. In this case GM will have a very good argument to absolve themselves as responsible: since you modified their wiring, how can they assume responsability for something over which they have no control? Besides, they sent you a letter saying it won't work!
Now that being said, if you take it upon yourself to modify the wiring harness, and you are confident that you are doing the right things, and it is the equivalent item after modification, and you are not worried about being able to sue someone if something happens, then you may very well go ahead and do it yourself. There are plenty of people who modify their trucks in pretty drastic ways. This is relatively minor in comparison.
You have to decide on your own if this is something you want to do, and accept responsability for it.
Others have posted on what wires to swap to get this to work. A search should reveal this. One instance is here:
Installing Trailer Brake Controller. So that we can understand what is going on, here are the pinouts for that connector on a 2002:
Relay Block - Body - C5
| Pin | | | Color | | | Circuit # | | | Function |
| A | | | LT BLU | | | 1620 | | | Trailer CHMSL Supply Voltage |
| B | | | RED | | | 142 | | | Battery Positive Voltage |
| C | | | DK BLU | | | 47 | | | Trailer Auxiliary Supply Voltage |
| D | | | BRN | | | 2409 | | | Interior Park Lamps Supply Voltage |
| E | | | --- | | | --- | | | Not Used |
| F | | | BLK | | | 1850 | | | Ground |
Translation:
- A is the signal to the controller that says the brakes are on.
- B is the supply voltage for the controller
- C is the signal from the controller to the trailer's brakes
- D is for controller control panel illumination
- F is ground
The positions of the pins are as follows, looking into the end of the plug with the wires going away from you, and the locking tab on top:
Code:
| Latch |
+---+---+
| D | C |
+---+---+
| E | B |
+---+---+
| F | A |
+---+---+
I am a little confused by the post I linked to above. It states that you have to move the red wire from one corner to another. According to this chart, the red wire is in one of the center positions not a corner. I have compared this chart and diagram against my 2002 harness, and it matches. This information is correct to the best of my knowledge.
If someone has this information for a 2003, please post it, then we can compare.
Important note: This information is only for informational purposes, so that we can understand what is going on, and the reasons for this letter from GM. Please do not take this as a recommendation to perform any modifications. As pointed out by CraigF in the following post, this is a critical saftey system. I will accept no liability for the use/misuse of this information. Any use of this information is strictly at your own risk.
-- SS
Congratulations on your new truck!
