Finally it's fixed, as mentioned before it was the wire to the low speed resistor that was the problem. The wire leading to the resistor on the top was completely burnt up and barely connected. I cut and cleaned the wire, crimped on a new connection and it solved the problem. When I first turned the A/C on and waited for the fans to come on something around the resistor started to smoke but then stopped in about a minute, I think it was just burn off of the left over burnt wiring. Now all seems well and the A/C works much better in traffic and slow driving.

Thanks to all who replied.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500