Ok, open the trunk with the key. On the inside of the trunk just below the latch there is a little access door that you can pop out with a screwdriver. Behind that door is the vacuum pod for the trunk lock.
The pod has two vacuum lines running to it. Both lines are yellow, one has a green stripe, and one has a red stripe. When air is sucked out of the red striped line, the pod locks the trunk. When air is sucked out of the green striped line, the pod unlocks the trunk.
You can test the pod by pulling the yellow lines out from the black rubber hoses, and sucking on the hosed individually. Each one should move the lock rod, and then should not allow any more air to pass through. If they continue to allow air in, then that is a vacuum leak and the pod should be replaced. They are available from Fastlane at the top of the page here.
Of course you can test these vacuum elements by mouth as I've described here, but there's a handy little device called a MityVac which is much better for the job. It has a gauge to measure the vacuum, so you can connect it, pump down the vacuum, and watch the gauge for a leak. You can pick one up at Autozone for around $25.
Let us know what you find out.
82 300D Euro
12 Honda Pilot
14 Tesla Model S
Former MB: 86 190E 2.3, 87 300E, 80 240D
Chain Elongation References