My 1996 VW Golf has a similar system built into the airbox, which when cold pulls the air in from a baffle over the exhaust manifold (through tube 2 in picture below). When the intake air is warm enough (switch takes place between 35 and 45 degrees C) the little door moves to open up the cold air tube (4). It's activated by a thermostatically controlled vacuum switch (3) at the throttle body header operating a vacuum actuator (1), which moves the little door. Maybe you could use that for switching.
