Its mounted behind the seat on the drivers side...Never seen something like it before...But I'm new to trucks so that should tell ya something...

 

It looks like a custom setup for air pressure. Look under the back of the truck for air shocks. If you have them it looks like they set up this custom gauge to read air pressure in the shocks. Below the gauge is the valve to put the air in. I did something similiar to this in a vehicle I used to own.

 

sweet