My rig didn't have one (engine poorly swapped in from a 73). I have the OP sensor but not a switch. I assumed that was factory. Looking at the 79 diagram I see it now. Rockauto has the original switch for $5.
Thanks for the sanity check.
79 Cherokee Chief, barely runs
(Cherokee Build Thread)
11 Nissan Pathfinder Silver Edition 4x4
09 Mazdaspeed3 Grand Touring