If the rear screen is cracked/smashed, you will not be able to use a suction cup to remove it. The suction could would not seal because of the cracks. I tried masking tape, 3M shiny HVAC tape, and others, and the suction could would not seal to those surfaces either.
I ended up using the sharp point of the smudger to remove some shards of glass to get underneath the cover to leverage it up and out. Definitely use gloves and safety glasses if you have to do this.