Why are you surprised at the $600 cost of a wiring harness for a 600SEL?
That's exactly why these cars depreciate so badly - their repair and service costs are very high and remain so long after their high status is long gone. Car's have no senior discount.
If the electric fans operate, the wiring harness, which you will need to replace, has nothing to do with the engine's overheating. That's assuming it really is overheating - that the temperature gauge is correct.
You don't say when the engine overheats. If in slow traffic, the radiator and fans are suspect but if it runs hot at speed the radiator is the culprit.
To ensure the radiator is shedding all the heat you should be sure its core is clean so the air can pass through it. If the radiator is doubtful you cal have it rebuilt. see:
If the radiator is OK, the fans run, there are no air locks in the cooling system and the gauge is reporting the engine's temperature correctly then you must consider the possibility of a busted head gasket. If that's the case, $600 will be the down payment for the repair job.