2007 International 4400 Wiring Diagram Request

Okay, so the latest issue with my truck is that the brake lights stopped working. My truck was built as an ambulance/rescue vehicle. Before I bought it, someone converted it to a tradesman bed (like a utility bed but with shorter sides to support a gooseneck trailer). Whoever put the bed on did a really half-### hack job on every aspect... including the wiring. When I bought the truck a year or two ago, the brake lights worked. They quit working a quite a few months ago and I did a quick fuse check. It's a 15a mini fuse under the hood on the drivers side near the brake booster. The fuse was good. If I remember correctly, the fuse only got power when you stepped on the brake. As a precaution, I bought a new Bosch brake pedal switch and installed that. That's as far as I got on that attempt. I drive the truck maybe once or twice a month. I actually forgot about the issue until a friend of mine pointed it out recently. "Oh yeah, forgot about that." So this weekend I really dived into the wiring. The first thing I did was check the fuse again. Now the fuse has no power going to it with the pedal pressed or not. I removed the fuse and stuck my tester down into the blades that grip the fuse leg and still nothing. I swapped the relay with the marker lights relay (which I know works because the marker lights all work). Still nothing. I left the brake relay in the marker lights and the marker lights still work. I creepered under the truck and started tracing wires. Wow, what a hack mess. I undid a bunch of black tape bundles, redid connections that looked flaky, redid some wire looming, rerouted some wires, zip tied wires away from danger points (hot or spinning parts) and just got a better understanding of what was what. The end result is that everything is now cleaned up and everything still works except the brake lights.

Typically, and I expect this truck is no exception, the brake lights are full 12v versions of the lower voltage parking lights. Since the parking lights work (and the turning signals), it's should be safe to assume the wiring, the bulbs and everything in-between is good. So I'm back to something on the cab side is not signaling (pushing voltage) to the bed wiring.

This is an air brake vehicle. When you push the brake pedal, you can hear a pump or something around the area of the master brake booster. This is where the limit of my air brake experience will begin to show. Should an air brake vehicle have a master brake booster with brake fluid? Is this a air-over-hydraulic system of some sort? While I was under the truck poking around, I noticed that the lines going to the rear brakes look like hydraulic lines. I did see a huge servo-looking thing (one on each side actually) that had a push rod sticking out the back that actuated something, maybe a fluid reservoir? Would the system go from hydraulic on the pedal side to air then back to hydraulic on the brake side? The brakes work great. Heck, I pulled a 30K trailer from the rental place with a man-lift on it before remembering my brake lights (aka my trailer brake signal) wasn't working. This beast certainly has enough braking even without trailer brakes.

The bigger question is what drives the power to the fuse that controls the brake lights? I ran my tester down the rest of the fuses and every one had power except that one. I tested my brake pedal switch and it is obviously working. Not only does the switch operate as expected via my tester, but it causes an action when you press on the pedal. I tried feeding 12v into the fuse output leg and pressing the brake pedal, but no brake light.

I briefly considered just running a new line from the brake pedal, all the way out to the bed wiring. This seems like overkill since I likely have everything I need in place, there is just one small piece missing.

Thanks for suggestions.