Sierra X,
Since you have existing AT/SLT led lighting, you will not need a Gen5 harness. You will need new 2016 style connectors (see ebay link in thread) to fit the new 2016 headlights. After you buy the new style connectors, you can take apart your 2015 headlight connectors and repin (re-arrange) the wiring into the new connector like Zmnypit did in post #135 in this thread (see his picture). Reassemble and you should be done...Note, the issue with the DRL not cutting out when signaling is still being sorted out.