Third/high brake light not working. All other lights good, tested lite & works manually.

If the high-mount brake lite doesn't work, voltage circuit. I'd run a new wire just for testing, for testing, it doesn't matter how you route it. Just to see if high-mount brake lite will work. If it works then you can worry about routing the wire so it is out of the way. In my opinion it doesn't matter how you route the new wire.
The body info on your vehicle should tell you how to remove this or that panel.
I may not have given you the answer you wanted? If so, My bad.

https://s26.postimg.org/s7ph40vm1/chm.jpg

Click on the link.
Wiring diagram for center high mount. Check voltage and ground. The white wire is voltage and the black wire is ground. The voltage comes from bcm. Use a test light, while somebody steps on the brake or use a heavy object on brake pedal. If there is voltage, the test light will light up. To check ground, black wire, with test light, Have test lite touching battery voltage, touch the black wire terminal, if the lite comes on, at least the ground is good enough to lite the test light. If you have good voltage and ground then I suspect the lamp assembly is faulty.

Your turn and brake lite use the same bulb filament, just not at the same time. I'm talking about lower brake lites in case you have a high mount brake lite, I don't think you do.
So if the turn signal on that side works ok, the wiring circuit and ground should be ok. The lower brake lite is wired through turn signal switch. The voltage wire for your brake lite is yellow, use a test lite to check for voltage with brake activated.

The cargo light will only work if the lights in the cab are on. Turn them on then flip the switch for the cargo light and it should come on, if not then it is probably an electrical problem.

Check all your ground points. The brake lights and cruise may be grounded to the same spot. Re-check the cruise fuse and brake/tail light fuses.

The third or High Mounted Brake Light runs straight off the brake light switch on the brake pedal arm. You say you tested the "brake unit"? I assume you mean the third brake light assembly, and if you get power there when someone steps on the brake pedal, then the bulbs are bad.

The power flow for the brake lights goes like this: Fuse ->Brake switch -> Turn signal switch -> Rear lights. The only difference is, if you have a 3rd brake light in your rear window, that gets its power directly from the brake switch. It does not go thru the signal switch first. So, if you are having a brake light problem, and your fuses are good, check the 3rd light in the rear window, if it lights up (assuming the bulbs are good, no one ever changes them), then the problem is most likely in the turn signal switch, hope this helps!

Since the high mounted brake light works you know that the brake light switch is working. Now you need to check the brake light bulbs in the rear tail light assemblies. The brake lights are separate from the tail light (parking or running lights). There are two screws that hold the rear light assemblies in place.

I just Know that you replaced the bulb ,try a test lite ,make shure it's grounded well poke inside the socket for the bulb this metal socket gives the bulb both electric and a ground ,this is the problem,good luck.

Most likely a short in your break line. You might also check the brake switch, but I would think if the break switch was going, it you wouldn't even have the third lite working...