Ramiro,
You have to go into the mirror harness - it IS NOT available at the kick panel as is mentioned in this thread. If you get logged into the ADC Mobile website (you now have access to all the tech materials), you will need to go tho the "Avalanche/SUV" section of the Camera Support page. Here's what it says:
You may have a mirror that has no easily accessible plug in the kick panel location to get the camera’s output from. In this case, you can always go to the back of the mirror to tap into the camera video positive and negative wires. These wires will be in pins 6 (usually white or pink) & 7 (usually blue) in the plug on the back of the mirror – you can also grab them further up the harness under the short overhead console – they are usually wrapped in blue foil further up the harness under the tape.
You just need to run a single RCA cable up there, chop off the end, separate the center conductor from the copper shield wire, then connect the center conductor wire of the RCA to the WHITE (or PINK) wire in pin 6 of the mirror plug and the copper shield wire of the RCA to the BLUE wire in the pin 7 position.
Walt