you should remove the existing adhesive then reapply using w/ 3M auto trim acrylic doublesided foam adhesive tape (can be found in most auto parts stores - even national chains). It's a grey tape w/ red covering - get the 3/4" wide version. Clean both the underside of the sill cover and the sill itself w/ denatured alcohol or a good wax and grease remover first. Then, lay two stripes of it on the underside of the sill cover (w/ about 1" or so around the edges) leaving the protective covering on. Then, place it where you want and mark the location using some blue painters tape. Then flip it back over, remove the protective cover and place it back. Press firmly in place for a minute or two and you should be good to go.
Good luck
