Not sure what model you have by my 300D has the door lock vacuum hoses for
the left rear door running from "Y" connectors under the driver's floor mat
back to the post and then into the door. Not much trouble getting inside the
door and you should be able to fish a new hose like you would an electrical
wire. Or, simply find out where the break is and, if you can get to it,
simply re-connect the broken line with a short piece of vacuum hose.

The vacuum hose goes down in the door post, reaches the floor, turns to
front then turns to right in front of both front seats.  Near the right
of car and under the carpet, a 3-way vacuum connector leads this hose to
the pump under the back seat.  The other way on this connector goes to
the right front door.

There may be another rubber connector at the turn in front of the driver
seat.  I know 85+ cars have it (which is a 3-way) since the passenger
door can also be used to control all doors.  84 cannot.

To remove the trim on the middle door post (B-pillar), move front seat
forward, loose the screw at the bottom.  The bottom cover can now be
removed.  I don't recall if the bottom trim of door opening is on top of
this cover.  If so, pry them up a little bit.  Now, move the main trim
on the B-pillar down to loose it.  Then you will see the vacuum hose.