5500 Integrated Electric Brake Controller Issues!!

[KSell5500]

2019 5500 purchased used. ... I'll start by saying; I've never had the controller function properly. Everything appears to function properly. Dash changes over to display electric brake output when the lever is pulled. Output changes on the display as (I would think) it should as brakes are applied. etc...
The issue is NO: output power to brakes, trailer connection/disconnect communication.
Troubleshooting thus far:
*I've toned the wire from the trailer plug into the cab at the ITBM. Signal seems weak but present.
*Disconnected the ITBM while truck is running and did receive the "service system" message.
*Replaced ITBM with new a unit resulting in NO change
*Connected brake force meter with NO connection to ECU, NO acknowledgment from truck, and NO brake output. All other lights/connection functionality present. Connected to both goose and bumper plugs with identical results.
*Driven around with brake force meter plugged into both goose and bumper plugs at speeds of 45+ mph with no intermittent messages concerning trailer connection/disconnection or changes to electric brake output.
Attempted to connect two trailers with identical results. Both trailers have had NO issues with electric brakes. Understanding this is the first vehicle with an integrated controller and reading about sensitivity to grounding systems, I added a dedicated ground to the trailer frame at each wheel with no change in controller functionality. I also added a ground from the junction box directly off the ground stud from the trailer plug to the trailer frame with no changes.

Anything of help here would be greatly appreciated. Schematics of the system to assist in troubleshooting (throwing more money at) components (electronics that I can't test)... anything....

 

[BossHogg]

It seems you've covered all the bases so there is something that is being missed. Trying to troubleshoot this on a forum is nearly impossible. I didn't see where you checked a fuse for the ITBM.

Measuring the braking signal out of the 7-way should show a voltage when the ITBM slider is fully slid over, it may not be 12 volts depending on the sampling rate of your meter since the brake signal is pulse width modulated.

The test trailers and the brake force meter used, have they been confirmed operational on another truck?

 

[KSell5500]

Could be wrong here, but I would imagine if the ITBM had a blown fuse it wouldn't provide an error when disconnected from the truck.

The trailers have both been utilized with other vehicles. The goose trailer that I updated the grounds on has actually been connected to another brand vehicle with integrated brakes and functions perfectly fine.

When it comes to measuring voltage... I thought of this as a futile task considering I replaced the ITBM with a new unit and had no change in performance.

I did check fuses, but unsure if there's some fuse "hidden" under some code name hence why I'm requesting an electrical diagram to aid in troubleshooting.

 

[chopperman1]

Is it original to the truck or did you add it?

 

[KSell5500]

Original to the truck.

 

[KSell5500]

Is it original to the truck or did you add it?

The truck was purchased used. As far as I know this is original to the truck. This truck was equipped by the original owner with a flat bed body including a goose-neck hitch, and a heavy duty frame welded bumper hitch. This is a factory cab and chassis truck. I couldn't imagine the original owner would up-fit this truck with two towing points and never utilize it for towing.

 

[KSell5500]

As a follow-up to very few responses at all... a bit disappointing.

I ended up tearing everything out of the truck to test from the module back.
I pulled everything apart on the dash and outside of the truck to gain visibility and physical access to EVER SINGLE section of wire leading all the way back.
I pulled apart, and tested, at every connection point. Cleaned, greased, and reconnected post testing.
I worked my way to the rear of the truck finding no fault or issue at all. That is until I reached the rear section where the factory original blunt cut wires were. The body builder installed a plug on the blunt cut wires to connect a split harness to feed the bumper and 5th wheel plugs. The body builder installed harness had a fault of some sort in it. Disconnecting that harness and connecting directly to the factory blunt cut harness resolved all issues. Unfortunately this harness is tucked between the fuel tank and flatbed preventing access or visibility.

I will be installing a new split harness soon...