In several places in the car there are pipes for heating and ventilation.
This is a PAP (paper/aluminium/paper) hose like Volkswagen uses originally. Thanks to the inner and outer paper envelope, the warm air is retained for longer (insulated). With aluminium only, hot air would be released outside the pipe. As the entire heating system is based on warm air and has to travel a long distance, heat losses must be minimised.
Under the Other Models tab, other cardboard hose diameters (PAP) are listed. If the pipe of this diameter is available in another material, you can find it under the alternatives tab.
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