With the switch being wired to a factory harness, that only leaves 3 possible options, 1) dealer option, 2) after market install w/oem looking switch/harness, or 3) the interior and wiring harness has been swapped out w/a 4-Runner's.  Anyone know where the 1st gen Runner rear window defroster switch is located? It has been about 5yrs since I restored one, and I can't remember if it is located on the dash or on the center console.

One way to know for sure if the switch actually went to anything would be see if there is another harness or cut wires along the top or bottom of the rear window.

Do you know where the truck was originally sold?

You might try reposting this question in chitchat or the general forum, to get more board members involved.