My mom had this problem on her Lexus also. After reading all the replies here and the fact that I could hear the relay or switch in rear window trying to work, I did this:
Got one of my thin plastic auto body trim tools and ran it around underneath the rubber window seal from the outside. Instantly fixed it with no door slamming. I figure as someone else here said that soap residue from the car wash could be the culprit. I suspect it was. Literally 15 seconds to fix. A person could use a credit card or other slim piece of stiff plastic to do this. No special tools required.