A common S13 complaint is that the climate control “always blows hot air”
In most cases it’s because the interior temp sensor has been unplugged while installing a stereo and the climate control module thinks the car is always at sub zero temperatures – so it can just be a matter of plugging it back in. Sometimes though, the sensor is actually faulty, or the climate control module has a hard time keeping the temperature constant (many of these units are over 20 years old now!) which results in annoying blasts of hot and cold air.
In my car, the heater would always run flat out until the interior temperature was up to 20+°C and then settle down and work ok, but the heater valve motor would continue making small adjustments and annoying noises.
I have now replaced the temp sensor with a fixed resistor, making the climate control think the car is always at 25°C - so if the temp slider is above that you get hot air, and below gives cold. Essentially it works the same as a manual heater control, which is much less annoying than the automatic system!