2014? 388 brake light problem

Is the light on as brightly as normal brake lights or just ghosting? Sometimes led lights will be dimly lit from the chassis node. Normal operation.
So wiring is as follows, turn stalk module to the CECU. CECU datalinked to the chassis node. Chassis node has about 7 or so powers coming from the fuse box on the firewall. Chassis node then sends out the signal to appropriate light. Normal rub spots for chassis node power wiring are by the drivers cab mount bracket, the P clamps right after, and then when it wraps around the bottom side of the frame by the batt box.

Right rear turn/stop is powered by fuse G3. G3 also powers left front turn.
If you pulled all the fuses from the cab that eliminates the CECU.
All you have left is shorted wiring to power or defective chassis node.
On the chassis node, white connector, pin 2 is the right turn/stop. Ohm it out to G3 power which is pin 1. If wiring checks out probably failed chassis node.
Chassis node can get swapped between trucks, same programming.

Like I said up top, ghosting of the taillights is normal. If incandescent bulbs are installed the ghosting goes away. Last 6 of the s/n if you want some diagrams

 

Left rear brake light works perfectly?

With the key off and the white connector unplugged, what is the ohm reading between pin 1 and 2 on the harness?

Im assuming your .1 reading was measured on the node vs the harness?

 

So if the g3 power wire and the rr turn wire are not shorted together, power must be going through the chassis node. I think it is safe to rule out a brake signal from the cab as both rear lights would come on.
I think it is looking like a chassis node failure, but if you have another truck available it would be best to swap chassis nodes first to verify. If not, i would check the main chassis node ground, connector a, black, pin 3. Load it with a headlight to verify its condition. I would also check the mega fuse mounted on the RH side of the batt box. They love to collect corrosion and the rear cable often snaps off. Give both cables going to it a decent tug and also check the rest of the battery cables.
I also attached the chassis wiring diagram.

 

Glad to hear its fixed. Chassis nodes seem weird until you statt working with them everyday. Takes a couple hard problems to wrap your head around them. I still believe 388/389s are some of the best trucks on the road. Easy to service, tough trucks. Not too shabby in the looks department either.

 

I have a 2012 388 my brake lights dont work most of the time then will work for a little while if I park with the lights working by the time I come back the brake lights dont work on truck or trailer and it doesn't matter if the truck is on or off can it be the cecu?

 

It could. Most likely not.
Good starting point would be the brake switch....