I live in the frozen state of MN. I can drive my 1996 Toyota Camry for half an hour but my heater works only part-time for some reason. The colder it is outside, the less it will work. If it gets above 25-30ºF outside the heat will be good enough, though not ideal. There is an option to have intake air come in from outside or recirculate within the car. However, switching between these two doesn’t seem to make much of a difference in the amount of heat generated.
Something that may be related: over the last few months, I’ve noticed that when I first start driving, I can hear the sound of liquid splashing around somewhere in the dashboard. It sounds like it’s about to pour out somewhere, but there are no leaks anywhere I can see, and I don’t smell any engine coolant. Does running low on engine coolant affect the heater? Does a lack of coolant mean I’ll hear it splashing around somewhere?