Ok guys the issue with windows is as follows..
There are two types of windows A that sit on an external and internal u channel rubber i.e the ones Rob say at Vanfest which are cheaper to buy and fit like old style car windows in essense that use a big chunky rubber.......not the pretiest but functional
And there are flush fit ones that are essntially flsuh fitted to the space and bonded in
With the later flush type the issuses are more in how the two panel skins are held together rather than anything else as some sinlpy bond these with silkaflex or bonding agent which is not really sufficient to supply the strength the panel requires so as to not loose any rigidity in said panel and the sides of your vehicle thus if they are toi be fitted correctly around the aperture gap has absolutely GOT to be spot welded not silkaflexed which is the correct way to do it..
Hope that helps a bit to clear up how it "should" be done