[First c10]

If not mistaken it you have a light bulb out or replaced a build with a different wattage then the BCM senses a different resistance. Try stepping on the bike pedal slightly or just enough to turn the brake lights on. And try setting your cruise.
I installed a utility body on my 08 gmc 3500 and the box went from a 2 light tail light on the stock box to a 1 light tail light on the utility box.... Since that day my cruise does exactly what you stated in you post. I plan to change the bulb from a stock 4.7 watt to a 10 watt bulb and see if that fixes my problem. But since I can get my cruise to work with a little playing around. I haven't been in a hurry. Or just lazy. Hope this helps.