The pins inside the lock cylinder. It will make it so the lock never locks again, but will appear to be operational from the outside. If you're concerned about security it probably isn't a great idea.

Courtesy of asgal.com:

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

You only need to disconnect the vac lines from the trunk actuator. I need the controller for my 84SD and the lock works fine with the key. There was a part change between early 84 and 85.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

wouldn't this just make it so anything can be used to unlock the trunk?
you would still have to turn it to unlock the trunk. this only modifies the implement needed...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

The CD has this problem also, the locks have been unplugged.

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83 SD

84 CD

In addition to disconnecting the vacuum lines, it might also be necessary to manually move the actuator to the UNLOCK position. Securing the actuator in said position would be a good idea.

No. The vacuum mechanism has nothing to do with the lock cylinder. Note that the "button" can be depressed without a key, regardless of the vacuum lock position.