Here's a doozy for you!! I had the same problem in my falcon.
Check all your stop light bulbs!! 
Most cruise controls wont work if there's a stop light out, as they use the stop circuit to dis-engage the cruise when you hit the brakes, and if it "see's" a bulb out, it "thinks" it might not disengage when you brake, and therefore doesn't allow it to work........ (Note the dumbass speak, that's the only way I could understand the sparky when he explained it to me, so that's what you get too!!
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Cost me $126 to find that out..........
I have a 98 series 1 discovery with cruise control, unfortunately it doesnt go. The switch in the centre panel (main on/off control) lights up but nothing happens when I flick the set/resume switches. 
Any ideas out there so I can once again 
. Have only just bought it x Japan so dont know if it has gone in the past and the dealer who sold it to me is making me 
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. To the moderator, sorry I am newish and didnt know the best place to put a post to get the best coverage.
