So, 1990 legacy wagon, I think the engine has about 150k...just started smelling radiator fluid, don't see any leaks anywhere, fluid is the correct color, but here is the kicker, when the car sits at idle, the air coming out of the heater gets cooler. I have no issues with the car overheating...any ideas? Head gasket going???

Hopefully the trouble is just due to an air pocket and not the headgaskets. You might try burping the coolant system and also do a block check to check for exhaust gases in the coolant.

Well if you can smell it you have a leak. Sometimes leaks are very small and don't show up because of all the oil and grease on the engine. Common places are the thermostat gasket, water pump gasket, and head gaskets.

Look for spots on the engine where the grease looks chocolaty/milky rather than just straight up black.