Need opinions on rear window install in a 72 Dart

I had a window guy come out and install my back glass he installed the window with the gasket on it, roped in, I had bought a rear window gasket from the Paddock right before they closed, window guy said the gasket was a really poor gasket, anyways we had to use butel to fill in the gasket under the window in spots, worked real well for about 3 years. Now I have a leak at the top of my rear window, I will be removing the back window this winter and having a new gasket installed and fixing the trim clips.