On the 109 series as well as most other MB chassis, the lock assy can easily be removed by opening the door and removing the 1 or 2 screws at the back edge of the door. once the screw or screws are removed, slide the handle towards the front of the car about 10mm and pull it straight out. You may have to tap it a little if it hasn't been out for awhile. You can now spray lube to your hearts content. Don't just concentrate on the handle; spray the latch mechanism inside the door.
PS: Models after 1980 or so require you insert the key in the lock and turn to remove. Antitheft.
Auto Zentral Ltd.