Yes it's the pressure relief for the truck to relieve pressure when you shut the doors. The original was smaller and wouldn't allow the doors to shut good. Be careful, I think I would try to get it installed somewhere else or you might not be able to shut the doors, and or damage something else. We started building S10's one year and the valve was too small and the rear windows would basically blow out from the pressure. Needless to say they changed them to a bigger one.
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