Demand call on a 2004 train heat pump split system for the upstairs. Found a thermostat wires going into the unit were heavily corroded I needed to cut all wires out and form new connections. System is operating at this time. While I was in the air handler I found the evaporator coil to be extremely corroded in my opinion the system should be replaced due to using r22 refrigerant on top of the cost it will be to replace that coil. Customer would like to let the system run until it fails.