Yep- - - -the diagram shows spring clip nuts that accept sheet metal screws and slip over the body metal back there where the pigtail lead comes through the bed. Any auto body shop will have plenty of them in stock, or a professional-quality parts store like NAPA would be another source. I used to buy them in boxes of 100 when I ran a body shop.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!