The actual vacuum switching happens at the headlight switch switch in the dash. You can disconnect the vacuum line behind the headlights at the actuator and then turn on the headlights. The actuators look like little UFOs. You should feel vacuum at the vacuum line you pulled off the actuators. If not go backwards from there until you find the problem. There is a lot of vacuum line in the system including a large tomato juice can that serves as a storage for vacuum. Just keep following the vacuum line back from the actuators. If there IS vacuum at the actuators the actuator itself is probably bad. You can manually release them from the parked position and see if that helps. If it does the actuator vacuum diaphragm is probably leaking. Use a vacuum pump like a MityVac to see if they hold a vacuum. Hopefully JohnRR will chime in here and give you a better procedure.
