Hello. I have a NavPilot 711 with a FAP_7002 processor unit and two FAP-7011 control units. The display is worn out on one of the control units and I would like to replace them. I do not understand the difference between the FAP-7011c and the FAP-7011c-a. What is the difference? Can I install two FAP-7011c or do I need one FAP-7011c and one FAP-7011c-a?