I have a rear start option on mine. It is a box with two toggles switches on it, the box is mounted next to the engine, mine is drivers side above the primary fuel filter.
If you were working back in the engine compartment and accidentally hit the toggles switch that says Front/Rear and hit it to rear it won't start from the front. I've done this two times, first time took me a while to figure out, not as bad the second time.
There are a couple relays in that box also that do the work of switching from front to back. Sometimes the terminals get corroded.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi