Try this, reseal the rear window using Butyl, its what ford uses to seal windows and so does my shop i work in, its real simple you remove the 8 nuts from the window that are behind all the interior and pop it out. Then take off all the old sealant clean the surfaces with rubbing alcohol then apply the new sealant (Butyl or how ever its spelt) then place the window back in and tighten the window in sequentially and dont over torque the nuts or you will have to replace the window. Then once in place alow one day for it to cure or completely seal before gettin it wet aka car wash. The while your at it you should have some left over cause it comes in a roll about 12ft long, remove your 3rd brake light and apply it to the light to make sure it doesnt leak.
If you live in an area were you get alot of precipitation do what i did take electrical tape cause it wont ruin your paint and put it around your window to alow it to stay dry while it cures then remove it later
hope that helps
R.I.P. 1999 Ranger XLT 3.0, 206,000 stock drivetrain
-Totaled after accident 2009.10.30 with brand new engine and tranny
1998 Ranger XLT 4X4 4.0, 200,xxx stock drivetrain
-Still running like a champ