You will need to remove the bumpers them selves.
Its a PITA task.
You will need some WD-40 and some duck bill type pliers. Needle nose, and 45degree needle nose too. A pry bar or big screw driver also.
Once you get the bumpers off you can flip them over and look inside and see a bunch of tabs poking through the bumpers. Soak them in WD-40, and dont be shy.
Start at one end, work the pry bar between the cover and the metal bumper and put a little tension on it. Not too much, you dont want to rip the tabs off.
With the pressure on the cover, start squeezing the tabs one by one. Just walk your way across the whole bumper.
Good luck.