Pardon me for speaking from total ignorance, but it would seem to me that for actuating the trunk lock, you would want the car to run the vacuum motor for a couple of minutes after the ignition is shut off, if the vacuum is low. This would be the series connection of the NO contact of a delay-on-off relay (McmasterCarr pn 6964K2-12VDC $61) with the contact of a low vacuum switch (Ford 81 granada vacuum switch connected to ignition computer).You would need a fused constant 12vDC wire to the relay power input, an ignition switch signal to the relay trigger, and connect the output of the vacuum switch to the vacuum motor hot. Of course, junk is much cheaper.