The 928 door windows have a scraper seal that closes the space between glass and the door assembly. It is there to prevent water from seeping into the door and guide the window glass sliding up and down when needed. However... these seals deteriorate, get hard and brittle over time. This results in water seeping into the door (possibly even damaging the motor assembly that moves the window up/down) and alowing debris and sand to lodge and thus scratch the window. The seals were horendeously expensive at Porsche. Something like 180 euro each, so 360 euro per set. We supply an aftermarket "remake" of the seal at almost a third of the original seal price. We sell them only as a set (Left + Right). Fitting is easy and no special skill or tools are required. An instruction can be found on my website. There is even a youtube video. Stock is limited !!