To replace a rear back window, detach any wiper from the window, remove the old gasket and window, clean the window frame, insert a new gasket and window in place of the old ones, and replace the wiper. Be sure to apply sealant to the new gasket for proper attachment.
To remove the wiper, pull it up away from the window, and use a flat-head screwdriver to loosen the screw that holds the wiper in place before detaching the wiper from the window. Cut off any damaged rubber gasket from the window with a sharp knife, taking care to avoid damaging the window glass in case the glass is still serviceable. Once the gasket is off, carefully detach the window from the frame, and set it aside.
Before inserting a new window into the frame, use a cloth to clean the frame with a strong cleaning agent. Once the window is clean, apply sealant to the new gasket's bottom edge, and round the gasket with a nylon cord, leaving a 1-foot cord on each end and securing the cord in place with petroleum jelly. Insert the new window into the frame, and pull the end of the nylon cord from inside the window to secure the rubber in place. For proper sticking of the rubber, have somebody else press against the rubber while you pull the cord. Wipe off any excess sealant, and reattach the wiper as needed.