As you go over the mountains hauling a tow, you might wish to energize those two huge fans MB placed on the front of the radiator. The factory scheme is to activate them as required by the air conditioning compressor freon head pressure sensor. I'd prefer not to have the AC on as I climbed a steep grade as the AC radiator is in front of the transmission fluid radiator and produces a lot of heat. Besides, I want to turn the fans on to cool the transmission before starting up the grade.
First thing is to replace the typically blown 30 amp fuse with a 40 amp but forget about the make-work factory wiring upgrade as my 40 amp hasn't blown yet.
Wiring is so complicated on this beast that I go for the easy work around. Pull the relay, pop the cover, drill the cover, and solder a stiff lead to the contact junction such that it will fit through the drilled hole. This lead needs to be grounded to activate the fans so I put a switch and an additional relay at the knock-out panel alongside the ignition switch. The ignition switch energizes the extra relay which provides the ground for the switch. The fans are loud enough that you probably wouldn't leave them on after turning off the ignition but the extra relay guarantees that another operator will not make this mistake.
I got through the firewall to route the wires just above the carpet behind the brake pedal with a sharpened length of 1/2" rebar from the engine side: just a tap to dimple and check location then a whack and you're through the unexpectedly soft firewall material.

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