It depends on what's wrong with it.
As you probabbly know, there are many buttons on your dash, but they are seldom the problem. For instance, there's a light switch. The lights are not in the switch, they're on the front of the car. There's also a wiper switch. The wipers are not in the switch, they're on the windshield.
Cruise control is the same way. The cruise control has a mechanical governor that is connected to the speedometer cable (not the speedometer) and that is in turn connected to the engine. All that stuff is under the hood.
The way to avoid being taken by mechanics is to leave it alone and just use the gas pedal. It would seem that trying to fix that particular option on a 30 year old truck is a pretty extravagant decision.