frost build up on the back wall

Hi. Is the unit still cooling properly? Heavy frost on the back panel would indicate a defrost issue. That the unit is not self defrosting. You will need to test the defrost system parts. If you have more of a ball of ice, then this indicates a sealed system issue. You more than likely are low on refrigerant and have a leak in the system. I have attached a service manual to help assist you. Make sure your condenser is clean as well.