[Porsche944]

Yes, you need the switch.

The switch is set to operate at altitudes above about 4500 feet (if memory serves).

The purpose of the switch is to tell the DME that the fuel mixture needs to be adjusted for high altitude.

With out the switch the engine will run, but since the fuel mixture is "off" you may not be able to pass emissions testing.

Any decent dismantaler like 944 Ecology or Parts Heaven shoul be able to hook you up.