A car heater in winter is definitely something we depend on and when it begins to malfunction it can be a huge inconvenience. While you are waiting for an expert to respond, I have attached some helpful links below that may provide information to assist you with your question.
If there is rust in the overflow tank check the inlet and outlet hoses to the heater core. If one is hot and the other is luke warm the heater core is clogged. Unfortunately there is an issue with the 3.0 engines clogging with muck and debris from rust which clogs the passages and eventually migrates to the heater core where the customer notices the problem. Only solution is removing the engine and popping all the expansion plugs and power washing the inside of the block. Anything less and the problem will continue to reappear.