Generally you want the car to heat up to temp as quick as possible. The part you're talking about is called a thermostat. Some people have used a thermostat from other VW engines (Type 1 Beetle, etc) with some success.
There is also a version that many refer to as a "Mexican" thermostat, though it was never on Mexican beetles, I believe it's an aftermarket part from Brazil or Argentina. This part does not "fail-safe" but there are also no reports of them ever failing. Take it as you will.
The system is set up so that the thermostat fails in the "safe" position, a spring should open the flaps if the thermostat looses it's thermostat-ing ability.