Bottom keypad switch. 

NOTE: Removal of backing paper voids warranty. DO NOT peel the backing from the new overlay and/or keypad without electrically testing first. Connect the ribbon cable of the new overlay and/or keypad to the display electronics and test the part prior to removing the paper backing and complete installation. If you have the same problem as with the original part, please continue troubleshooting the unit to ensure the problem is the overlay / keypad.