2014 KW T660 No headlights and left turn

Have you pulled apart the DT connector on the frame and checked with a probe light?

 

I had a similar issue, but, at the rear of a newer T680. I had nothing at the rear of the truck, except 1 turn signal. I went through all the lighting circuit fuses, in both fuse boxes. After finding nothing blown, I started going through all the other fuses. I found a blown fuse on a circuit that had nothing to do with lights. I replaced the fuse and all the lights came back. CAN bus does some bizarre stuff.

 

Go back to your fuse boxes and start pulling the fuses, one by one, and use a 12V test light to test each side of the fuse socket. In CAN bus, you need to see voltage on both sides. You’ll want to take notes as to which sockets don’t have power on both sides, then, try and figure out commonality. I’ll just about bet you, there are no wiring problems. You’re thinking old school. Newer stuff doesn’t work that way, especially when what you’re experiencing doesn’t make logical sense like how we used to think.

 

16k outrageous. Another post on here where the dealer wanted to replace a harness or 2 for something like 10k.Took it to a place that specializes in electrical and repaired it for less than 1k.
Take a look at the turn signal switch. Just about everything runs through the chassis node, all the lights, wipers. I see in post #4 that you checked that. By doing what?
Check and clean any grounds from the headlight wiring going to the frame.

 

I’d sure want to see how they diagnosed it, especially for that kind of money. From what I know of dealerships, replacing an entire harness has become the standard MO, instead of actually finding the fault.