Always start with, Year, make and model. And eng trans if power train related. They all look the same until you open the doors and look under the hood.
If your using the relay to get brake lights, and they illuminated then the problem in the cab.
No brake lights and no cruise control, both are actuated and disconnected via the brake switch. Try checking or replacing.
We have FREE, OE service manuals in PDF format. Use search bar at top, " service manual" , look in the paragraphs , for the posting. Download your year, may be more than one volume, get all your year. Wwwwaaaayyyy better than any Haynes, Clymer, aftermarket.
Some of us have more than one truck. So always start with year, make, model, you'll get more answers.