Owners/service Manual problems of the 2009 Kia Motor RIO

Part affected:-door: the rear passenger door will not unlock, manually nor by key-fob, therefore I'm reduced to using three doors on a four door car. *this is the second time this malfunction has happened. The first time, it was the passenger front door. All-star kia of baton rouge repaired it because the car was still under warranty however, now that the car is paid for and no longer under warranty they want to charge a $100. 00 diagnostic to see what the problem might be because they don't know.