I've been told by Carl at Digital Dash Solutions that the 2015 Sierra WT (base model) does NOT have the needed connection wiring inside the dash?.. I have not yet confirmed this, but I will be looking into it since I have a WT that I want to add the backup camera too as well. Previously, I had spoken to someone at the local GMC dealer about flashing my stock HMI for a backup camera and they quoted me around $150 to do it, but did not mention any wiring issues. A wiring diagram should make this easier to determine if anyone knows where to find a WT specific diagram. Looking under my truck, I found the CA connection port with the filler plug inside, and it also had the pins inside the port after removing the plug. Maybe someone else knows more about the WT model?