Behind the charcoal canister is this rubber hose that is capped off. The hose leads to a metal tube that heads back. It gets lost in the maze of other tubes. I'm about to start tearing things out, I'm sure I'll find it then. But does anyone know off the top of their head?

 

Looks like a breather plug, maybe able to run it at a higher location for mudding or incase of submerging...

 

That's exactly what it is, a vent hose.

 

That hose you removed back near the transmission in your other thread probably connected to the breather on the original transmission which is likely connected into this line. When they put in the TH400, they forgot to connect it up.