It could be the brake light switch, which is like $11. Its a fairly common problem. For further troubleshooting refer to this thread with some good troubleshooting information.

Number one cause of a failed cruise control is the vacuum line running to the servo on the drivers side under the battery. Battery acid leaks down and eats a whole in it. Next check the brake light switch on the brake pedal. Seems to be the next most common failure and cheap replacement part at the auto parts store. The clock spring in the steering wheel also will go bad and is pricey. Also the cruise button on the steering wheel gets dirty contacts that can be cleaned.

http://www.dieseltruckresource.com/dev/cruise-control-not-working-t93341.html

http://www.dieseltruckresource.com/dev/cruise-control-not-working-t289140.html

Good luck and feel free to donate!!!

Answered by blaze45 (expert)
395 answers provided