1985 Mercedes Benz 500sec Battery is going dead over night

What I remember on that system
that it is a vacuum actuator
you have a vacuum pump and lines
so if its a locking system issue I would be looking at the vacuum pump and not the actuators as they are operated by vacuum and not by power from the battery
system pump is in the trunk by the spare tire well
check it out

This is a classic amperage draw that is being caused by a control unit not fully turning off after vehicle shut down. Here is how I have always attacked this issue on MB vehicles. First what is your static reading on your meter after the vehicle has been locked? Note that this measurement should be taken after 15-30 minutes of sitting locked. Most of the time amperage draws will be under 1 amp thus explaining why the vehicle goes dead over night or in a few days. Here's what I recommend as far as an order to diagnose this.
1. Role down all windows and lock the doors so that you can have access to the fuse panel under the drivers dash (this may require some acrobatics to get to with the door closed) Make sure everything is locked and off
2. Next open the hood and trunk and manually close the latches for both with a screw drvier (push the screw driver into the latch until it fully closes) Hood will have 2 latches trunk will have 1
3. Now with your meter connected in series between the negative battery post and cable terminal let the vehicle power down for 15-30 mins.
4. Once you have reached a static amperage reading go to your fuse boxes
5. Start pulling out fuses one at a time and after you pull each fuse check your meter.
6. What you are doing is trying to isolate the component circuits
7. Repeat this for every fuse in each fuse box one at a time (this is time consuming but it works)
8. What will eventually happen is when you pull the fuse that is for the component that is drawing your power the amperage draw you see on your meter will drop to appropriate levels. When this happens check what fuse you have pulled and the component it serves will be your faulty component that is drawing power. Replace the component serving this circuit and your draw should be gone.

Note that this often times will take hours to find but this is the only sure fire way to isolate an amperage draw. Also keep in mind only pull fuses not relays. Also when you plug some of the fuses back in you may reawaken the vehicle in which case you will have to reinstall the fuse and let the vehicle power down to static amperage.

Also let me know what has been replaced on the vehicle and any other repairs or circumstances that surround this issue. I would definately recommend not throwing parts at this though because most of the time this approach never works with these issues. An amperage draw is something that is measurable and any good tech will be able to tell you with absolute certainty if the problem is fixed or not because all you have to do is look at your meter to see if there is still a draw. Time and patience will be your best tool in this repair.