I do not know exactly how a W140 SE is build, but most mercedes models get power for the vanity mirrors through the front cabin courtesy lamp. The powercable for the vanity mirrors is (normally)connected by a plug in this light unit. If all features of this light unit are still working, it is possible that the previous owner took out the plug for the vanity mirrors to prevent them causing any electrical damage (since the screw shorted the power...).
Some of the mercedes models do have an additional fuse box in the trunk of the car! I do not know whether the S-class has one as well, but I suppose they do.
You can check whether the rear defroster is powered or not by taking out (or just pulling aside is enough) the upholstery in the back at the left side (vertical part) and checking the connection for the defroster. Normally this is the place where the powerplug is situated in the chassis and you can check it with a "proeflampje"...
with a minor 600+k