I have a 98 Durango that has 210.000 miles on it.About every two years when I start to notice lack of heat at idle I know its time to back flush the heater core.What I do is I disconnect the inlet hose to the core and loop it back to the water pump and then I drive down to the local car wash and using their spray guns I back flush the core through the cores outlet hose. I have probably back flushed the core 3 times and have had no issues yet with the core leaking.At some point I know I will have to replace the core but until that time comes I will just continue to flush it every two years or so..