The problem is in the rear audio/ hvac control unit. There is a technical bulletin regarding this matter that I have included for you below. You can disconnect the unit and reconnect it to restore operation temporarily. The permanant fix is to replace the rear controls.
Bulletin No.: 03-08-44-018C
Date: January 09, 2006
Subject:Rear Seat Audio (RSA) and/or Rear HVAC Inoperative (Replace RSA Module)
Models:2003-2006 Cadillac Escalade, Escalade ESV, Escalade EXT2003-2006 Chevrolet Avalanche, Silverado Crew Cab, Suburban, Tahoe2003-2006 GMC Sierra Crew Cab, Sierra Denali, Yukon, Yukon XL, Yukon Denali, Yukon Denali XL2003-2006 HUMMER H2
with Rear Seat Radio Control (RPO UK6)
This bulletin is being revised to add 2006 model year vehicles. Please discard Advanced SI Resolution Bulletin Number 03-08-44-018B (Section 08 - Body and Accessories).
Some customers may comment that the Rear Seat Audio (RSA) or the rear HVAC controls are inoperative. The display will be blank and there is no communication to the RSA/HVAC module.
A microcontroller was found to lock up causing an "ALDL No ALDL" communication condition. This required the cycling of power by disconnecting/reconnecting the RSA module.
Replace the RSA module to correct this condition. The RSA module has been updated to eliminate the "HVAC No ALDL" communication concern
Sorry for the delay in getting back to you. The module is the rear control itself. It is easy to replace,
Insert a flat bladed tool between the audio control and the console end panel in order to pry gently the audio control from the panel.
For pricing information, contact your dealer.