Not fitted ? or cannot be fitted ??
'Most' Disco series ii and facelift ones are 'pre-wired' for cruise and it is a real doodle to fit, takes less than an hour with no more tools than a couple of screwdrivers and a sharp knife/file to cut out the blank for the switch.
The easiest way to check, is to pull out the blanked binicle switch (RHS one up from the fuel flap release) and see if it has a plug and wires in the back, if it has, BINGO, your disco is wired for cruise control.
Steve