That seal is bonded to the quarter glass, the entire glass assembly is what you're after. That rubber does degrade and shrink over many years, the top edge is what usually breaks loose from the body and water comes in there. I have sealed the top edge of four of my Explorers there, using Ultra Black RTV. Just a light skim coat at the seam where the seal touches the paint above the window, that will stop most leaks. If you do that carefully, nobody will notice the RTV there, mine you have to look hard to see it. I used some green painters tape on the paint to leave a 1/4" gap between it and the seal, and wiped excess RTV off carefully with a finger.