As mentioned it is likely something the PO was using it to power something else(or planned to), and used the factory switch for the ease of putting a switch in an oem location. I have done something similar in the past.
To find out for sure pull the switch and see if it is attached to an oem harness or just has a spliced wire to the oem dash lights, or see if there is a harness end above or below the rear window.
Of all the 80's era pick-ups my friends and families members have had, and the ones that I have restored, I have never seen a defroster in a pick-up's rear window, there really isn't a need for one. If it is all factory it is the first that most(if any) of us have ever heard of one being in Toyota mini truck.
