In the rear passenger side wheel well, the height sensor is bolted to the body, and connected to the control arm with a small ball and socket link. Pop it off at the control arm. Moving it up should turn the compressor on, down should vent air. There will be a delay, so you'll have to hold it in each position before the system reacts.
If you position it too far up, it will set a code for the position sensor and the system won't inflate or vent.