Did you check for DC voltage as well as AC voltage? Check for 12 volts DC.
Only 2 wires, white and yellow = 12 volts. For AC prewire - there would be white black and bare or green, and likely 12 gauge not 14 gauge.
Looks like you are pre-wired for a 12 volt fantastic fan at that vent opening. Likely terminated on your 12 volt fuse panel in a position where there is wire laid down next to a fuse holder with currently no fuse in it, if you see no DC power.