I have seen this tube for years and it bugs me that it goes to some unknown place. It is actually broken at the mount but nothing comes out so I'm confused.

 

It's where the A/C drains off the condensation from inside the cab.

 

Wish it was the a/c drain cheap Ford just put a 1/4" of hose sticking out of the firewall and makes a mess running down the body and frame. Actually found this photo after like an hour of searching. I was on a mission to find out. It's the vent/overflow for the trans (4r70w) no idea why they made it so long and complicated.