2002 Chevy Silverado TOWING MODE

Going to need a scan tool. With the key on engine off, cycle the tow/haul switch while watching the T/H mode on the scan tool. What we want to know is if the scan tool sees Active when the switch is depressed, and Inactive when the switch is turned off. Since you have replaced the harness, I would still perform the electrical test in the diagnostic flow chart. If an electrical problem isn't found it could be a bad switch, or a problem with the BCM (Body Control Module).