Inside temp sensor part reference:
Conrad Best.-Nr.: 500586

R 25: 2.2 K Ohm
Type: B57164-K222-J
Tolerance: RN ±5/10 %
Version: K164/ 2,2K/5%
Operating Temperature: +25 °C
Temperature range: -55 up to +125 °C

Measure at the mixing flap between both yellow wires. You should see about 3,7 - 4,7kOhm. (control lever cold-warm range)
Armin
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The heating system uses an interior temp sensor to adjust heating automatically. Climate control so to speak. The interior sensor is in a little fan, located between the glove compartment and the center console. On top of the dash you see a little grill. That is where the sensor is in.

You can hear if the fan runs, and using some smoke you will see that it draws in air to measure the temperature.  When disconnecting the fan you can measure resistance of the sensor:

Inside Sensor: NTC
20 Degrees C = 2570 Ohm
25 Degrees C = 2220 Ohm
30 Degrees C = 1900 Ohm
Carefully clean the resistor if needed.