When the lights are turned on or come on, we get transmission fault and the third brake light comes on but the regular brake lights do not work. Changed battery, brake light switch and I've searched through the wires under the passenger threshold and cannot find the splice I've been told about (but am planning to search more). Any thoughts appreciated!
forgot to mention the third brake light stays on.

 

Does it have a trailer harness?

 

yes but I completely unhooked it today and it didn't work.
it worked for a long time before this started.