I have an 04 GMC Sierra 2500 HD 6.0 and I desperately need to know where the release cable and the other cable that connects to that one is located

 

The parking brake release cable is under the dashboard on the driver’s side. Look near the parking brake pedal. It connects to a second cable that runs toward the back of the truck. That cable goes to the parking brake mechanism near the rear wheels. If you follow the first cable from the release handle, you should find where it connects.