Bad ECU???

On a 98 and on that lights up no matter position....when that knob is set all the way to defrost

 

That appears to be one out of a truck that didn't even have AC cuz the buttons missing. But if you actually look at the selector mechanism for vent selection it doesn't have any switches in it to tell if you're on a position that would be considered defrost. In other words it's entirely up to the operator of the truck to determine if the AC is on or off

 

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I'm confused about the point you're trying to make but yeah that's the AC button.