Are the two fans by the radiator running when the car has been on a drive i.e. the engine is hot?
I have a similar issue with my golf MK4 and after lots of research and investigating have narrowed it down to the fan control unit or its related sensors. Since my compressor is not getting its 12Volt signal and the fans are not running it looks like it is the fan control unit. The unit is called the j293 in documentation and its part number is 1J0 919 506K. It drives the compressor clutch and the radiator fans and takes input from various sensors. In the golf the Unit is underneath the battery i.e. it is bolted to the battery tray that the battery sits on.
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