It appears that for some reason there is not hot coolant going through the heater core. I would start by making sure the monovalve is functioning properly and the aux pump is working properly. If not, then there is always a possibility that the heater core is blocked, but this usually will only happen from neglected coolant changes.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record