Boost Pressure (MAP) Sensor
This is the Boost Pressure Sensor, commonly also referred to as the "MAP" (Manifold Absolute Pressure) sensor. It slots into the top of the driver's side "Elephant Trunk" hoses (as seen here) and measures the pressure of the air passing through said hoses.
While very similar to the air pressure sensor (located on the airbox), this is a different part number from that sensor.
For 2004-2006 vans ONLY.