Now, that you mention that the speedometer works, then I believe the MPH signal is actually getting to the CAT ECM and going up to the speedometer(again assuming only one MPH sensor at rear of tranny). 

Earlier posts make me believe that the cruise on/off works, the set/resume works, the clutch and brake switches work, SO now, I wonder about ECM programming ; because if is programmed to 127 mph at Low Cruise=cruise will never turn ON. (This setting is for companies that don't want their drivers using cruise control  )

Typical cruise programming = low cruise at 20-30 MPH and high cruise at 70-127 MPH (note: 127 MPH is the default=doesn't mean it goes that fast)


You may need to have somone with a computer(with CAT ET program on it) look at this truck and trouble-shoot it; IF SO, make them print out all the ECM parameters, so you have a record of how it is programmed. Also, ask him to program it how he normally sees a truck that runs in your application would be programmed (=get rid of programming from old company specs, that the engine was used for, previously) 


Note: At same time, with a computer hooked up, have them test jake switches operation, and look at jake programming, too. They can also test the wiring, up to the jakes solenoids to see if the current makes the jake solenoid plungers go up and down(must remove valve covers) AND with valve covers off, then at idle, they can manual push on jake plungers, to see if jakes work (=are they getting oil pressure to them) That procedure throws a little oil around, and is just a little messy.