According to my memory (and more importantly, double checked against my original window sticker!) the block heater was part of the Cummins engine package. This included 136 amp alternator and some other stuff too. Look on the right side of the radiator support where the plug should be located.
On the 2003 and later, it is offered as an option. The heating element is always there, but the cord is optional (might be able to buy it after the fact from Cummins or elsewhere. )