[GMCPaul]

Every time we've seen this the strap located under the vent window assembly that joins the inner and outer door skin together is broken and has allowed the outer door skin to spread apart from the inner door skin. Remove the vent window assembly and you'll see the metal strap underneath it and I'm 99% certain this strap is broken. A sign of this is the rubber is closer to the door glass at the rear of window opening and further away from door glass up by the vent window assembly. These straps are very thick and repeated opening and closing of the vent window wears at them and they eventually break allowing the inner and outer door skin gap to widen.
Paul Jr @ GMCPauls