Return to the lines under the plastic cover at the outside edge of the driver's side footwell.

The red line that is highlighted travels straight back in to the rear driver's side door.

Pull this line from the "Y" connector, attach your vacuum pump and draw a vacuum of 10 to 15Hg on this side of the system. If the rear driver's side door is unlocked, doing this should lock it.

Watch the gauge for a short time and look for vacuum leaking off.

If there is a leak, you will need to pull the panel from the rear driver's side door to test the line itself. To do this, you'll disconnect the red line from the actuator and plug it off like you did the lines of the master switch.

If the line holds vacuum, move on to test the lock side of the actuator. To do this, connect your vacuum pump to the connector on the actuator that the red line was connected to and draw a 10 to 15Hg vacuum and watch for leaks. Make note of the results and continue.