There's 2 relays all ready on the firewall, 1 has the gray wire for the pull side. Can I use the existing relays? The pull side is 40 amps and the hold side 10 amp draw. I have to turn down loads going over Eisenhower Tunnel and Vail Pass. I pull a 9k trailer and some loads over 13k. I thought that I could get by without buying a new BD or Pac Brake system. What comes inside the cab? Guess you've figured it out that I don't know jack about it. Thanks George in Denver.