Robert Plant wrote the lyrics, which were a response to critics who claimed their previous album, Led Zeppelin III, wasn’t really rock and roll. Led Zeppelin III had more of an acoustic folk sound, and Plant wanted to prove they could still rock out.
This was the first Led Zeppelin song used in a commercial. Cadillac used it to kick off a new advertising campaign in 2002 with the tagline “Breakthrough.” The company was going for a hip, new image, since their audience was slowly dying off. The spots aired for the first time on the Super Bowl, and sales rose 16% the next year.
The Bag Lady 
I’m surprised that Zep were okay with it, they used to tell advertisers to go away.
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