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Discussion Starter #1
I'm trying to find out whether a jumper harness exists, from any source or manufacturer, to cleanly connect a trailer brake controller to a 1998 GMC GMT400 (without RPO UY7 or Z82).

The problem is, I don't have the truck... I'm helping a 75 year old friend remotely.

Since millions of GMT400 trucks were made, I didn't think it would be such a problem, but all of my search returns end up showing the GMT800 underdash electrical connection and jumper harness.

I've seen a couple of images and videos where people have just stuck a wire in there, jury rig style, but I would prefer to recommend, at any cost, a plug in style connector that does not require fabrication, so that my 75 year old friend can just plug it in where it is supposed to go, like I have done on my GMT800.

So, if anyone with a GMT400 from the late nineties has a trailer brake controller, can you please talk about how your trailer brake controller is connected?

Thank you!
 

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I don't recall any GMT460 being controller ready.
 

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Discussion Starter #4
Update:

So far, I have talked my friend through digging very deep into the factory wire harness to locate and retrieve the heavy gauge blue wire and orange wire that were very well buried inside the harness at the rear of the vehicle (as far as the rear axle) and at the front of the vehicle (tucked underneath power brake booster.) These are the two high current power circuits, that GM must not have been sophisticated enough in the mid 90's to thrift out of production. One harness for all, whether equipped or not equipped with the option. I wouldn't expect to find this today, some 25 years later, now that automotive just in time manufacturing methods are vastly more aided by computerization so as not to waste the copper.

But I suspected that 25 years ago, it was cheaper not to have so many different part numbers for each vehicle option nuance, and I was right. All the heavy duty towing wires were there inside the full body length chassis harness... even in this 10706 light duty model. I also counseled my friend to find the 5 wire light duty harness wrapped upon itself and clipped up to the rear quarter panel. So, we have 7 wires for towing, running front to back, all GM.

Now, my question is about the BLUE wire.

I seem to recall in my GMT800 (pre prepped for a brake controller with the mere addition of a jumper harness), that there was a POST in the Underhood Electrical Center for the BLUE output of the trailer brake controller. But, I don't have access to my GMT800 at the moment to verify this.

The present predicament involves the GMT400's two posts, Post A and Post B, in the underhood electrical center. Each appears to be fused with a 30 amp fuse. Each also appears to be unused.

My plan is to advise my friend to attach the orange wire to one or the other of those two posts.

My question is does it make a difference which post?

Both posts meter the same, KOEO, KOER, and no key at all (constant hot).
 
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