If I'm understanding things correctly, the cold brake wire is basically the brake light wire (12V when pressing on brake, 0V when not pressing brake) but you want to access it either in the cabin or under the hood rather than running a wire all the way from the tail light up to where you are mounting the control box. Is this correct?

Have you looked in the download section? There is an electrical troubleshooting document posted there. Look on page 110-8 & see if that gets you what you need.