Kris
I would like to get a few things straight in my mind.

First you said

"fan preresistor caught something on fire.
As I understand it the resistor is only used for "low" speed which is only kicked on by the air conditioning pressure switch trigger.
Presently my ac is not charged so this isn't really a huge issue but I am curious."

Why would the preresistor get to hot if it wasn't being used? Did you have the A/C pressure switch jumpered?

Did your car come with the duel fans?
If it did, then you may have other problems like a bad fan (Drawing too many ams) or something.

Now look at the diagram you supplied. Look at the wire from the preresistor to the fans. It is a 2.5mm wire that splits off to 2- 1.5mm wires. That makes me think the MB thought that a 2.5mm wire was big enough to run both fans at low or high speed. I know the length of the wire comes into play but generally wires are rated for up to 10 feet in the automotive field. Larger wires if more than 10 ft.

Second
I am having a problem with the 200A CB tripping. Where in the circuit is this CB installed? Unless you have added a lot of power consuming devices, you should never see a 200A load. The only time that much would be used is while the starter is engaged and the engine not running and the starter usually is never fused. If your car is drawing 200A when not starting, then you have a fire looking for a place to happen. In over 50 years of working on cars I have never seen anything like that.

Could you set my mind at ease and help me understand how these things are happening?

Thanks
PaulM

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84 500 SEL (307,xxx miles)