Mix up dish soap and water 50/50 in a spray bottle. Put the new gasket on the rear window perimeter. Spray the new rubber gasket liberally on the edge that installs into the metal window frame. Then wrap a string (I use a heavy shoestring for work boots or hiking boots) around the perimeter of the gasket. This is where you need a second set of hands. Have someone line up the rear window with the gasket and string in the window opening from the bed side of the truck. With the other person in the cab, pull the two loose ends of the string , which will cause the rubber gasket to,the inside of the window frame. Gently pull the string while the other person continues to apply pressure to the rear glass. Even pressure needs to be applied as the gasket inner lip is pulled inwards. Dont press too hard and dont press the center of the glass. It takes practice and patience, but the string trick has always worked best for,me. If the window isnt centered in the frame, start over...use a liberal amount of soapy water.
...and dont don't drop the window. I also suggest stuffing a towel in the space between the cab and bed so the widow wont drop between or break.
