I don't know what colors your vacuum lines are, but on my 73 280 SEL the lock line is white and the unlock is black. Since it isn't unlocking, I start with checking the unlock line and connections in the trunk, then the trunk actautor. Then the "T" for the tunk line, which should be in the area of the center post, before the right rear door, it could be the "T" or the connectors. Then move on to the actuators in the doors, possibly a leaking/blown diaphram on the unlock side.
I just went through this with my system, it is time consuming, sometimes tight quarters and often frustrating as you keep chasing leaks. A quality vacuum gauge or hand held vacuum pump will assist you in finding the leak.
73 280 SEL