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I believe it's in the small area in the top/middle of the dash, 4 or 5 small slots out by themselves. Cover that area with your hand for a while and see if they come on.
I would think it would come on sooner not later, though, if being dirty was the problem.
Yes, it is in the middle of the dash under a small grill cut into the dash. If it was dirty, they would come on all the time or at least early. I found this out by getting a dash mat without nthe cut out. Couldn't figure why the lights stayed on during the day. Located the problem and cut the dash mat and now all is well. This is on our '01 F150 S Cab 4X4.