I did a search and found a few dozen very long threads that didn't really answer my question... or they got hijacked halfway through

I have a new-to-me 06 Express AWD passenger van. Confirmed by RPO that it has the towing package. I learned (from the search) where the trailer wiring harness plug is, so I assume that part is behind the taillight.

What are my chances of having the harness under the dash for a brake controller? I don't mind wiring it up myself, but it sure would be nice to not be laying on my back in the snow right now.

 

In case it matters, it has what appears to be a factory hitch. At least it just has a generic sticker on it instead of a brand name. I doesn't have a 7-pole plug on it, nor do I immediately see where one might have been removed.

First couple photos are under the dash over near the parking brake. I found a bundle of wires taped off with the white shiny label, but no connector. The labels on the sticker are too degraded to read. Second photo is where the rear driver's side frame rail meets the bumper. The colors look right for trailer wiring... except there is one that appears to be lime green. Are these my huckleberries? I was of course hoping for a weatherpak connector, but I won't look a gift horse in the mouth if these are the right wires.

 

Looking at the pics your provided combined with experience from my own van you appear to have some kind of towing prep package since you don't have the actually 7 pin connector and hitch that the full towing package comes with these days on the Express. The blunt cut wires coming out of the rear frame rail in the back are likely your 7 wires to wire up a 7 pin. Use a test light to verify their functions. The wires under the dash you pictured do appear to be the factory blunt cut wires for tow prep for the brake controller. Again use a test light to see if they function and what they do. You should have a ground, posiitive battery feed and a wire that is only hot when the brake pedal is pressed.

In the case of my 2019 Express with no towing RPO options, I had the same wires under my driver side floor as you but they did not have power and further investigation into the under hood fuse center showed that those circuits were blank and not present. I didn't have any wires coming from my rear frame rail like you do which leads me to believe the wires under your driver side dash will be ready to go.

I had to wire my complete system from scratch but it looks like all you need to do is buy a generic 7 way pin and a brake controller and make your connections.

 

I was wondering if the wires at the back used to have the round factory splice point and someone chopped them off. I find it hard to believe that GM would leave a 10-ga 40-amp fused wire stuffed in the sheet metal with just a couple wraps of electrical tape.

Once the snow melts I'll get after it with a test light and see what I come up with. Does anyone have a wiring diagram? Testing will obviously do the trick, but the wire colors are a bit abnormal for GM. I see the typical yellow, green, brown, blue, but there are a couple mixed in that don't seem typical, like the lime green with what appears to be a brown or dk red stripe.

 

I just wired my 2019 for both 4 and 7 pin, and controller, etc. Are those wires by the pedal 4 wires exactly, kinda hard to tell. Also, the label looks almost legible, maybe worth a second look. clearly just an electrical tester will answer all questions. Controller needs hot, ground, brake signal, and wire to the rear. Finally, are there only 6 wires in the rear bundle or 7? Clearly you know rear is 7, but pic seems to show 6. For everyone else the 7 are: 1. Hot, 2, Ground, 3. Left turn/left brake, 4. right turn/right brake, 5. reverse/accessory, 6. controller signal, 7. running lights. Again a multimeter, or even a 12 volt light tester will tell you what's already working.

 

In truth, I was laying on my back in the snow to get that rear shot. By the time I took the under-dash picture I was wet and cold, so I didn't take the time to count while I was there.

It's getting inspected so it's at the shop now, but I'll get to it with a multimeter. I'm guessing (hoping) there is no fuse in the box for these wires. Electrical tape isn't a very good long-term insulation.

 

Update.... finally got some time to dive into this.

I tested the wires under the dash. The label was toast, but I was able to make out RLR AU which I figured was TRLR AUX and another that said CHM-something which looked like CHMSL. Meter testing confirmed a hot, ground, brake light. Got the brake controller on and working.

I tested the wires in the frame (also found a wiring schematic on GM upfitter) and they all do what they're supposed to do. The frustration is that GM seemed to grab whatever wire color they had for the 12v power and ground wires. Mine has the red/blk for the 12v+. Good news is, I can tow now with signals and brakes.

Bad news is, no matter what I do, that red/blk never has power at the back. I pulled both the mini and maxi fuses and tested continuity, and verified 12v at the pins in the fuse block. According to the wiring schematic, power goes Batt > fuse > X88 trailer connector (blunt cut). None of the schematics show it going through the BCM, a relay, or anything else. I have power in the circuit at the fuse panel, but nothing at the back. I thought maybe it needed key-on, or it got tripped by a relay from the BCM, or a relay that the ECM triggers after it sensed the engine was running.... but nothing.

Before I go doing harness surgery looking for a break, is there something obvious I'm missing? I read somewhere that there is a pair of blue/red wires near the fuse box?

 

Hi , do you remember what the RPO code is ?

Thanks

 

Codes related to "tow package" (which is a totally nebulous term)
UY7 WIRING HARNESS TRUCK TRAILER, HD. This can mean anything from "we put some extra wires in, good luck finding them" up to a pre-installed 7-pole plug.
VR4 TRAILER HITCH WEIGHT DISTRIBUTING PLATFORM. This is the option that physically puts a platform hitch on the vehicle prior to delivery. GM usually used Reese
Z82 TRAILER PROVISIONS SPECIAL EQUIPMENT, H.D. This is generally considered the RPO that indicates tow package
Z85 CHASSIS PACKAGE INCREASED CAPACITY. This is an upgraded capacity rear suspension for a heavier tow rating. I don't think it was ever used on the Express/Savanna
KNP COOLING SYSTEM TRANS, HD. This added a transmission cooler, but I don't think it was ever used in vans.

 

I’m not sure on the vans, but for the trucks, the red/black wire that provides battery/charging voltage to the 7 way is either tucked into the looming of the harness leaving the fuse block or it’s tucked underneath the lower cover. It has to be connected to one of the studs on the fuse block to be powered up.

I don’t think the vans have studs on the fuse block but maybe the wire is still inside the looming and just needs an inline fuse to provide power.