How do I remove defrost vent on dash?

I am trying to replace plastic defrost vent and tubes ,there are 3 flat nuts on the under side I cannot get a ratchet on them any ideas would be appreciated 65 dart w/air

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When all else seems to fail I have a 4 inch crescent wrench I use for things like that.
Yea, it's a pain and takes patience but it works.
On that broken vent I would start snapping it off in small pieces from the top side.