I'm sure you've taken care of the problem by now, but other folks may be interested in a reply. Best I can tell from your photo, you needed a new windshield gasket as the existing one looks worn with age and ripped on the edge. It should fit in the channel securely unless rust has mucked up the fit. If the windshield channel is messed up, you'll want to first arrest the rust and apply rust inhibitor. You can then use 3M Black Super Weatherstrip Adhesive to make sure the new gasket stays in the groove.