What they said. Typically found only on some '72's and '73's, along with a switch on the side of the manual transmissions.
I think it's on the driver's side, so you should be able to see some wires running up to the trans underneath.
Since this is a '72, if the VIN has a "P" or a "Q" in the sixth position, post up your information in Viperwolf1's "Mind Your P's and Q's" thread.
An example of the appropriate VIN would be U15GLPxxxxx and we're looking for late-'72 and early-'73 stuff. Like factory auto trans, light bars on the dash, special fuel return fitting up near the carb along with it's six-port tank selector switch (as opposed to the standard three-port type) and other stuff like that.
Oh, along with what month the truck was built in, assuming you still have your driver's door decal intact. Many have been lost or painted over, over the decades.
Enjoy your new rig.
Paul