I have a 1986 300sdl that had some pretty strange electrical problems when I first purchased it. Specifically, the power door locks did not work, and the exit light under the doors did not operate and the fasten seat belt light kept staying on . I found that the drivers side rear exit light shorted out and blew the fuse. When I replaced the little light assembly under the drivers side rear door and replaced the fuse, my exit lights all worked under the doors and my power door locks now worked. BUT my fatsen seat belt light would continually flash, and the main inside light by the sunroof would not turn on when you opened the drivers side door, but it would with the passenger side door. Also my seat memory buttons would not operate unless the doors were closed.
I have the factory manual for electrics and studied and studied the wiring diagrams and decided that the interior light by the sunroof panel was bad. I found a nice one on e-bay installed it and still had the same problem. Also I then realized that when I started the car with the seat belt off, no warning buzzer. Also no warning buzzer when I left the headlights on and turned off the car and opened the drivers door. I then looked under the dash on the drivers side and found the SEATBELT WARNING RELAY. It sits on a Bracket just above your feet. It is bolted to the Cruise Control Unit. I took it off and opened it and found a bad solder trail on the circuit board. In fact it had melted. I traced the circuit board functions using the service manual and decided I needed a new one. WOW, they are expensive. I then bypassed the actual circuit on the board with a jumper wire I soldered between the two connections. I plugged it in and guess what????

When I open the drivers door my interior light comes on. When I put the key in the ignition, the warning buzzer sounds until I buckle my belt. When I leave the headlights on and turn off the car and open the door, a buzzer sounds. And Now my seats work from the memory button with the doors open.

I looked at the wiring diagram again and realized this probably all happened from the shorting out of the rear exit light.

I re-installed the relay and all is well 9 months later with no problems. I keep looking for a wrecked 300sdl to see if I can find another relay as a spare, or another one that has not been damaged, but no luck yet.
This may be the cause of all of your problems.
Good Luck and keep us posted.