That trim has a little "spring" to it. Gently (but with some meat) pull the bottom away, and lift it up. You may wanna put a strip of 2" wide tape above the trim. When it lets go it may want to jump up. Dont be surprised and anything you find back there! Its not unlikely that the discs that were screwed into the sheet metal and the screw holes will be rotted out (at least somewhat!). Bill