Про этимологию - upfitter - это человек, который будет его использовать при дооснащении. Просто название, подобно Jacob's ladder или butcher knife, который все же не "нож мясника" по-русски:-)

http://mysite.verizon.net/deviouscustoms1/install.html
If you want to add a few aftermarket options to your new Ford Super Duty, Ford Motor Company makes things easy for you. One of the options you can select from the factory is the "UPFITTER SWITCH" option. This is a bank of four switches, with fuses and associated relays, that provide two 30 amp and two 10 amp circuits. With the upfitter switch option, you can add a winch, lights, strobes, two way radios, or whatever else powered by the truck’s electrical system you can think up. There are many other uses for these switches that other users have developed, so don’t be afraid of asking for ideas!

Вот и подтверждение - это возможность дооснащения после покупки:
http://72.14.209.104/search?q=cache:NT4qTT87dT8J:www.fleet.f...
SEIC (Stationary Elevated Idle Control)APCM (Auxiliary Powertrain Control Module) and theassociated Auxiliary Idle Control Option (Option Code96P for F-Series, or 961 for E-Series) are no longer available. a. The SEIC (Stationary Elevated Idle Control) fea-tures of the APCM are included in the PCM(Powertrain Control Module) of all F-250/350/450/550 and E-Series over 8500lb. GVWRb. *****Upfitter interface is a set of blunt-cut wires above parking brake release handle*****
c. Upfitters will need to provide their own user interface

А вот как этот set of wires пограмотнее назвать - это уже в следующем вопросе, и не ко мне:-)

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