headlights out!

Are you saying you have no low beams and the high beam flickers?

The wiggling the harness is a good clue that's where the problem is. Wires do get hot and melt the insulation for different reasons.

If the blue light is on it sounds like a short but do the easy checks first.

First I would check the ground wire where it bolts to the frame and make sure it's good and clean. You'll have to remove the bolt and clean the lug and frame. You should be able to follow the black wire off the headlight. It shouldn't travel far.

Also check your relays. You have a high beam and low beam. Stick the high beam relay in the low beam slot and see if that makes a difference. They are the same part #.

If it's neither of them then it's either the harness or high beam or headlight switch.

 

I had an issue in the past with a T660 where there was a short in the wiring harness right near the headlight that melted the insulation and caused similar problems. It wasn't easy to spot the problem but cheap quick fix once we did.

 

I have had breaks in the wiring harness where they bend when raising the hood if not there I would change the turn signal/dimmer switch.

 

Yes one of the prongs is a ground. I think there's a very good chance you have a break in the ground wire where the hood hinges or where the harness has been stepped on a lot along the frame where drivers stand to clean the windshield. When using a test light it will show that your getting fire, when your not really, this is because your usually grounding the test light to a good outside source. Try checking for power using the ground in the plug, I think the slot on top is the ground(I think) not the two that run the same way(I hope this makes sense), you may have to attach a male spade connector to each end of a short piece of electrical wire to plug into the plug(JUMPER) to use the test light, if your test light doesn't light up you've got a broken ground wire. Don't forget that Kenworth uses ground switches instead of traditional hot switches, which can be a little confusing sometimes. Hope this helps.