Tractor protection Valve

The air lines and such were removed form my truck as I don't use it commercial.I am installing a air over hydraulics system to work my disk brakes on my 5th wheel camper. At the protection valve is a red 3/8" line and it supplies air when trailer vale is engaged. Been using it for air for tools for a while now. Trying to locate my signal line. I do have a large 5/8" line run to rear of truck capped off. It being so large makes me think it is for something else.My truck has been singled.

 

Ok, it is my line for the blue connection. Done some digging on DTNA site and found it. Wasn't where I thought it should be though reason I couldn't find it before.