Does your valve look like this?

Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI

If you don't need heat, you could always install a shut-off valve on the line going into the heater valve.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible