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I beleive you are refering to an AIC (Auxillary Idle Control). These are available from FMC when you order the truck for $210, if you didn't order one with the truck you can get it at the parts counter but it is about $500 over the counter. There are some after market units out there made by alarm companies and such that also make remote starting units.
The AIC is used to up the idle speed for extended idling (longer than 10 minutes), to prevent wet stacking. Deisel engines run extremely cool when idling for long periods which causes imcomplete burning of the fuel, leading to deposits on the cylinder walls and valve stems, which can lead to sticking rings and valves. This then leads to broken engine parts.
AIC is also used to increase the idle if you are running auxillary items off of a PTO.
Do a search on this forum for AIC and Wet Stacking, lots of posts here to read.