2013 kw t660 help please

[Chamberpains]

If I'm reading that correctly, you're having a problem with your left rear tail light/parking light being dim. Correct?
If all your other lights are coming on correctly including the right tail light, then I would physically trace the wire from the tail light to its next connector. Check your voltage there. It should be 12 volts. If you have 12 volts there then there is a broken wire from that point back to the tail light. You will have to physically trace it out by probably cutting the harness loose and visually checking for rubbed, bulged or exposed pieces of wire.