A very strange thing happened to my 86 300sdl yesterday. It has a 6 month old battery and it died yesterday while it was parked in my lot at work. Strange thing too, because it was a beautiful 75 degree sunny day.
I had to manually unlock the doors and the trunk, retrieved my jumper cables and hooked them up to my friend's Ford F150. As I hooked up the jumpers, I noticed my aux fan was turning. Everything in the car was off. When I tried to start the car, it would barely turn over, but the aux fan was turning at high speed. I opened the fusebox and removed fuse number 1 and the aux fan shut off and the car started immediately.
I drove home and started to investigate what could be wrong. I dug out the electrical schematic and started to read. I put the fuse back in fuse number 1 and the fan immediately started to run at high speed. I located the aux fan relay in the fusebox and pulled it and the fan stopped. I plugged it back in and the fan started. I unplugged it again and replaced it with one of the other relays ( the one from the power windows) and the fan stayed off.
I believe that my relayy stuck in the on position ( probably old age) and my aux fan killed the battery.
Luckily I have a large stash of used parts and found another relay and plugged it in the power window relay location, charged up the battery overnight, and all seems well. I will monitor to see if the fan comes on again while the car is off.

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John
1986 300sdl
1985 380se