Here is photo of the temp sensor wire that comes out of the fusebox and goes to the front of the car. The temp sensor is located behind the front bumper (behind the license plate). It is possible that you don't have the sensor and the temp display is showing nothing. If you connect a 3k resistor instead of the temp sensor the display should show something like 20-30 degrees in celcius. Search the forum for the exact resistor values.
Good luck.

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1989 230E, 8v, 166.000 km, updated to 94/95 trunk & hood
2002 Daewoo Nexia 50.000 km
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1987 VW Jetta GLE 16V, Recaro seats
1982 Volvo 240 DL (lovely car!)
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