Got a bad case of the wiring demons....... Help please

Brakes should be on it’s own circuit.... from the brake light pressure switch.

 

I am not familiar with Sterlings. But if you can locate the pressure switch for the brake, it should be pretty apparent which wire is always hot and which isn’t. Then either trace it or run a new wire to the brake lights.

 

What's last six of VIN. See if I can't get you a wiring diagram.

 

Not much Sterling experience but being an L model I assume it is built off the old Ford line.
In general trucks have two brake light switches. One for the truck and one for the trailer. The one for the truck sends power to the turn signal switch. The switch has a green wire going to the right rear turn and a yellow wire to the left rear turn. When the switch is not on power from the brake switch goes through the turn signal switch to both tail lights. When the turn signal switch is turned on it disconnects brake switch power to that side and connects it to the turn signal flasher.
Sounds like maybe you have connected the wiring for a trailer, which is different, to the back of the truck.
Check to see that you have power to turn switch when you mash to brake and you are using the correct set of wires. I expect the tractor brake switch to be around the firewall.
Also all old school switches are not wired the same and I have had to swap some wires around in the new switches connector.

 

Green means stop? People that graduated the new math program are now designing wiring harnesses?
Excuse me living in the past. Harnesses tend to be all the same color wire anymore and have circuit boards in everything.
Hope you get a wiring diagram and that helps you out.
Power probes are a great tool for some of these problems when used properly.
Maybe just put four lights on the back like a trailer. brakes in the middle and turns on the outside. No wiring problems?

 

Can't find way to upload stupid cgm type files... Freightliner had to make it hard. Here are wire numbers and colors. LH turn wire #283 yellow with black stripe. RH turn wire #282 green. Ground wire #57 black. License plate light #11 black with yellow stripe. Back-up lamp #140 black with red stripe. From your description of what is going on they used trailer connector instead of chasis wiring.