what would cause voltage in brake light wire

Hello, How many wires do you have for your brake light switch? You may have 2 or more, but cruise control depends on the brake light switch to cancel. So, if the brake light switch is constantly allowing partial voltage through, it will consistently disable cruise control. Make sure the brake light switch is getting 12v, and see if it is sending 12v only when pressed, and 0 volts at all other times.