The Rear Window on the Ford F-150 tends to leak after a few years do to the dust and weather. To fix the leaking rear window you need to remove the glass and then re-install it with fresh new Butyl tape.
This is do to a poor design that Ford had in the earlier F-150′s
CPR Auto Glass does this type of repair for around $85.00
Please comment if you have any questions or would like to attempt the removal of your window yourself.