Do I need to change the heater core? I have a 1997 Jeep TJ Wrangler and I just put a new rad in it. Blower works but it will only blow warm air for a few seconds and then it's cold. I tried a 'reverse' flush of the heater core but I did not get the 'gunk' that others have claimed fixed the issue. The water pump was changed a few years ago as well. Core or thermostat?