Welcome to the forum here. I deleted your other thread you started as it was the same, and I merged over FONZI's post here.
Let me try to answer your questions in order...
So you are leaking water from top nipple on the reservoir tank. The best advice i can give you is replace it with a new one. I do not mess with gluing it. Not worth the risk. I know one person who destroyed a nice v12 W140 Engine after rebuilding their own transmission all because they got cheap here. Buy a new one, or get one out of a junk car at the junk yard that is not leaking. Here is list of junk yards:
Row52 | Search Results
https://www.lkqpickyourpart.com/locations/
The heater problem, my simple advice is to re-insert the connection to the climate control unit there on the center console, or the vacuum vent controller, but.........The power lock pump could be getting water, or the vacuum line it shares vent controller could have a leak. Many possibilities. Also, in the engine bay near the passenger side are vacuum lines tied to the plenum box. Make sure all of that is connected...
What I would do since you are banging on the climate control. Is remove it, and re insert the the two connectors as maybe one of them came loose, but to do that on the 92/93 cars it is an "Act of Congress" to do. You need to remove the bottom center console which you need to pry the rectangular the shifter ring that may be metal or plastic. Once that is off, two screws will be exposed at the front bottom of the angled piece of the other center console. The factory radio will have to come off, and it may need some radio removal keys to remove it, but not sure. Then I think you can lift the top center console from the bottom. Then the climate control will have four screws on all corners, and you can slide the module out and re-connect the back rear connectors. This is not easy.... Now, the device that controls all the vents directly, and gets the signal from the climate control unit sits behind the glove box zip-tied to the steel cross member. That connector could be loose, or some vacuum line to it can have a bad connection, I'd look at that too.
The locks, You may have water where the door lock pump is at as the older cars are known to damage the circuit boards on those integral pumps. The pump is located under the passenger rear seat, and the pump can be rebuild/fixed. For the record I fix these pumps for many people here, but on this old style of pump do not waste your time removing each vacuum line. The pump has a key to remove the top manifold with all the lines attched... Here is how you do it:
http://www.benzworld.org/forums/w14...her-pse-connection-question.html#post10891746
You may have to clean the circuit board with contact cleaner, but what goes bad on these pumps is the motor, and most can fix these. Here are some threads...
http://www.benzworld.org/forums/w140-s-class/1585361-diy-pse-pump-post-mortem-resurrection.html
http://www.benzworld.org/forums/w140-s-class/1827546-pse-pump-needed.html
Sound System, Okay I rebuild these amps too, so you know I'm the resident Transmission Guy, and the one that rebuilds the amps and pumps. Now, what can be happening is the amplifier connector can have corrosion. You will also need to un plug and plug it a few times. To get to the amp is little challenging. The drivers side trunk paneling has to get removed as the amp is near the driver's side trunk hinge behind the flat paneling. The amp is on top one this goldish metal frame being held by four bolts. The corrosion can be from the CD changer connections too, but this CD changer on 92/93 cars is a big unit somewhere on the left or right side of the trunk. Now these old 92/93 have a separate front to back control volume control like device on the center console on the wood near the center arm rest. These are known to loose connection, get scratchy like volume control.
You can use Mercedes coolant with a UV dye to find leaks, or you can use Zerex G-05 from Napa Auto(they are cheaper than everyone else). Flush with only distilled water do not use tap water. Change the radiator drain valve as they known to leak and break... Change your thermostat and the o-ring as the o-ring gets crushed with time...
Engine oil, your owners manual should have the oil sheet with viscosity's, but most here are running full synthetic Mobile One. Here is a copy of the Oil sheet from my 96 straight six posted here on post 23:
http://www.benzworld.org/forums/w140-s-class/2056337-amazing-what-correct-weight-oil-can.html
Hope this helps,
Martin