When you unhook the factory head unit. You have the FM plug, the big gray and big black plug, and then you have a little black plug on a black wire. I had no place to hook in this little black plug on my new head unit i installed? was i supposed to hook it up? The head unit works fine.

 

I already replied to your post in the audio section, but that is the ground wire for the radio, right now you're probably grounding throgh the antenna, hook that wire to the black wire on the new radio.