i just bought a 1978 f250 it has a small rectangular light mounted on the bottom of the dash right above the transmission tunnel its a blue light that has the word lock written in black in it. it has a ford wire connector going to it and it does not light up with anything i do i am just trying to figure out what it is for i have never seen a light like this befor please help its not a big deal just driving me nuts lol

 

That's the indicator light for the full-time NP203 transfer case, it illuminates when you're "locked" into 4 wheel drive.

 

When these go out is it usually a bulb or switch issue?