Doesn't this go to the fog light I'm out here looking on my broke 04 Prius now but the front bumper cover is off I see it's tied in with the little sensor that goes on the hot water tank It even hangs down like it would be heading towards the fog light.
Isn't that for the little grill light on the left side of the car that's on the same harness with the sensor that goes into the coolant recovery tank that's below the headlight and when this red connector is dangling correctly it's aimed right down in the front area of the car where the bumper cover would be with the fog light or if not it's taped back and not used? I believe this is right I'm staring at it on my '05 Gen 2 right now
The electrical wiring diagram has the fast answers to that kind of question: Toyota Service Information and Where To Find It | PriusChat an example (for a different thing, of course) for how to look it up is in this post. Looks to me like the connector in that position with one light green and one white black-striped wire is indeed for the fog light.
