I own this car when was brand new, way back in 2007. Until about a month ago, everything was fine. When the temperature fell. I realized that there is no heat while at idle. As soon as you start moving the heater kicks in and makes it warm. But when you stop, even at Red Light, air becomes cold. Don’t know whats going on.
The low coolant is quite likely. Another question is whether the last month has just been colder than usual. Every car will have reduced coolant flow to the heater at idle. Use a quick responding infrared thermometer to actually measure the output temperature. It might be below body temp…still warm.