Missing Ground Wire?
 

I have a 1998 Dakota. I am going to install my Alpine headunit into it. I bought the dash kit and the wiring harness. But I notcied the wiring harness doesn't have a plug for the ground wire. All my other harnesses for cars, VW's, Mazda and Ford have all had a ground wire on the harness. My question is, did I get a faulty harness or do I just hook the ground up to something metal in behind the deck. TIA