[Bounty Hunter]

Could be the flex joint on the turbo up-pipe. Could also be the flex joint or flange between the DPF and the SCR. I would start looking there for exhaust leaks. Should be easy to follow the carbon deposits.

I also suggest replacing the 2 cab vents behind the rear seat.

 

[Bounty Hunter]

You can have a shop weld it, but you're welding compromised metal. Better off welding in a new flex joint.

 

[Bounty Hunter]

I was right on where to look for the leak. You should still consider replacing the 2 cab vents.

 

[Bounty Hunter]

I have never even looked at those cab events before. What will that accomplish by replacing them?

The exhaust made it into the cab somewhere, and that's a likely location. Water can leak through the vents as well. Cheap and easy to replace.