Greetings all, I've been on and off working on getting a 1997 C230 to reliable daily driver condition for the past few months. I've been trying to trace what seems like a leak in my vacuum system.

The first issue I found was the central locking not functioning at all either with the key fob or the button on the center dash. I first checked all fuses and ensured they hadn't popped (specifically fuse no.27 in my case) and found all to be just fine. Next, I located a few vacuum lines with the help of some forum posts to find that many were dry rotted/cracking, which was to be expected given the car is 27 years old, so I replaced those. Specifically I replaced the rubber fitting that connects the white vacuum line to the nipple on the back of the intake manifold, the line from the Secondary Air Valve to the Air Switching Valve, and then lastly the fitting on the Fuel Pressure Regulator. Whatever I did seemed to somewhat aid the central locks, however they function slowly/weakly, and the driver's rear lock doesn't function at all, could this be caused by a leak in that door or a general leak in the system? I haven't had the chance to take the door panel off to see for myself. Adding to that, locking and unlocking the doors from the center dash results in a prolonged humming coming from the pump in the trunk on the passenger side. From what I understand, this is also indicative of a leak in the system.

Secondly, my rear folding headrests also do not function at all. When pressing the switch on the dash, I can hear an audible *click* coming from the package tray area. I also understand they only work when the engine is running. I've tried pressing the switch multiple times in quick succession, holding it down for a few seconds, and pressing/holding it while having someone push on them. None of these solutions worked, so I took a look at the lines going to the folding mechanisms from inside the trunk (which are a PITA to see and get to by the way) and they all looked fine. So I'm honestly at a loss for what could be causing this issue.

I have some time over the next few days to get my hands dirty with this car so any info is greatly appreciated, I'll update this thread with what I try and what works/what doesn't. Thank you in advance.
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Have you checked the box in the boot on the right side, behind the side cover

 

I helped my neighbor with his door locks on his '83 300D.
Once you've done one, they're all pretty much the same for troubleshooting.
But you have to get the diagram to know what's in the system.
This is what I did:
1. go to the vacuum source and disconnect it from the pump or manifold or whatever
2. use a vacuum pump to evacuate the system
3. if the vacuum escapes, then you have a leak
4. reconnect that vacuum line to the source
5. start checking for vac leaks on each component based on common sense, that is, what seems to be the main source of the problem (you said the driver side rear door seems to be not working, so that's where I'd start).
6. you can approach it different ways. One way is to remove the door panel/card and check the vacuum on each component inside. If they all hold vacuum, then you know they're OK.
7. inspect and test the rubber hose connections. Sometimes they're cracked or decayed and leaking. Sometimes the hard plastic lines rub through and leak, but it's rare.
8. another way to check is to go to the area in the vehicle where the main line connects to each branch. Then test each branch with the vacuum pump/gauge for leaks. You can narrow down each branch that way by process of elimination. That's how I diagnosed my neighbor's. It was a PITA b/c I had to pull up the carpet on the driver and passenger door threshold to access the connections to branches.
9. I forgot to mention that if the HVAC actuators are on the same main vac branch, you have to separate them, so the HVAC vacuum system is not part of your tests. On my neighbor's 300D, the connections were outside the cab under the hood (mechanical vacuum pump as source). So I just disconnected the HVAC line and plugged it's source (it's a rubber 'Y', and was easy to disconnect and plug). After that it was easy to trace-down the leaks.
10. Keep eliminating vac branches until you find the one the leak is on, then go check each component on that branch.
My neighbor had 2 leaks in his system. One was the fuel fill access door release and the other was a bad rear door lock vac pod. We also replaced some of the rubber connectors between the vacuum pods and hard plastic lines, due to decay and cracking. Incidentally, you will find yourself removing panels, trim pieces, and pulling up carpets along the process. It's not for the 'faint of heart'.
Hope that helps give you and idea of how much work you have to do to figure this out.
Cheers...