1996 classic cruse issue

Ok i have fixed about every part on a truck other than a cruse control.....lol

Here is what i have.

a 96 classic detoit 12.7

the cruise will not work on the road. if you stop and set the high ideal it works fine till you turn the headlights on at which point it kills the cruise. freightline says alot of companies did this to keep drivers from using cruise at night when you"fall asleep easier" their words not mine. now that can be reprogramed at 175 dollars and hour,,,,lol however what would cause the cruise not to work on the road. and any ideas where to look for issues