WHERE ARE THE WHEEL SENSORS LOCATED ON MY 2008 GMC SIERRE 1500 THANKS

They are a part of the wheel bearings or hubs. Take a look at this article - > http://www.brakeandfrontend.com/issue/article.aspx?contentid=38809 -- You can also search for "Subject 05068C Front wheel sensor" -- it will lead to a JustAnswer site PDF that had diagrams of the sensor in earlier GMC's. Not everything may apply to your 2008 but it covers the general location and wiring harness.