When you turn the key on does it have power?
Was a power drivers seat an option way back then?
I do not believe it does.
I would check to make sure. It could be a power wire to a switch that was not installed at the factory based on whatever options the 4Runner came with. I know I have a few blanked out spots in the center console area. If I remember correctly I used one of them to power my CB.
When you follow it back is it part of the factory harness or does it look like someone spliced it in?