2007 6.7 3500 with manual trans:

I tried starting my truck yesterday morning when temp was around 30 deg F the engine won't crank. There is no audible click.
Later that evening I checked the battery voltage a little low around 12.3 VDC but it had been sitting for several days.
Then I tried starting it and it starts right up no issues multiple times...outside temp around 55F
The next morning it's again cold around 30 deg. Won't crank again no clicking.
So I jump it to my car to see if its a low voltage issue, still no start no audible click.

Any ideas where to start?
Does anyone have a wiring diagram for the start circuit?
I'm hoping there is a relay that is failing that the coil opens up when its cold.

 

Physically grab your cables and see if they crunch. Sometimes the cables look great physically, but be corroded inside.

There is typically a small wire going to your starter, that one should go to 12v when you turn the key. It causes the solenoid to close and the always hot large wire connects to the starter motor. See what is going on at that point, then trace it back to the problem. Could be a bad ground connection too, make sure you've got a good ground on the engine block.