I think some of the pricey high end caps have a tilt-in front window to let you do that if you crawl up inside, but the fact you are asking the question suggests that you don't have one of those. Me either. It's always been a problem on my old Dakota, and one of the reasons I didn't get a cap on the new 1500. I got one of the window squeegie things, thinking it might fit in there, but it didn't. Which is sort of okay since I needed it for front and side windows anyway. But reach in with bottle brush, rag on a stick, etc. Opening the back window helps for some of it. Power washer if you're careful with it.