Backgrounds are attached behind the content of whatever part of the page you have the background attached to. The z-index determines the layering of the content and not of the background.
I tried it, and it worked great as to going behind the header background, but it covered up the navigation....ohhh, a thought just occured to me, maybe I should give the navigation a z-index......
lol i know just enough to make me dangerous as they say....