Is your Ford a diesel?
Both our old truck and the current truck have plug-ins, in the engine compartment. We never used the one in the 93, nor have we used the one in the 99. I'm not even sure where the one in the 99 is, though my dad assured me it was there. (He never used it either.) Both trucks appeared to be built in Canada, so it isn't surprising.
Just a thought, anyway -