One day I noticed the heater would not produce any heat, not matter what the position of the thermostat.
It turned out the thermostat cable tied to the knob was disconnected behind the console.
Apparently this is a fairly common problem with this car and garages love it because it can be made to look like an expensive repair, such as A/C replacement, etc.
Instead I fixed it with just a couple screwdrivers and a little bit of time.
There's a lot of plastic covers to remove but in the end, I could find the connector that was indeed disconnected. It plugs up, so I suspect after 15 years, it slowly nudged itself down via gravity and vibrations.
Even though it's somewhat difficult to access with your fingers, I was able to plug it back up.
Works like a charm now!
Long story short, I compiled the few videos that helped me remove the plastic covers in order to get access to the console. Counter-intuitively, you have to start from the back seat to remove the center console and work your way towards the front.
Then follow the cable that connects the thermostat and the other end might be dislodged just enough that it cannot control the heater anymore.