From 1976 to 1978, the true inspiration for transforming cars ruled the Saturday morning airwaves: Wonderbug, from legendary TV creators Sid and Marty Krofft. Their Krofft Supershow brought you Land of the Lost, Electra Woman and Dyna Girl, and most importantly, Wonderbug.
Like any good superhero, Wonderbug had a secret identity: the Shlepcar (with a license plate that read SCHLEP), a jalopy cobbled together out of many different cars that looked sort of like a dune buggy. When his teen pals blew a secret horn, he became a sparkly dune buggy called Wonderbug (license plate 1DERBUG), with the power to fly. I think the credits above tell you volumes about how silly and awesome the show was. Obviously, 1980s show Transformers was a pale imitation, and Michael Bay has probably spent his entire career with the Transformers series trying to get back to the pure excellence of Wonderbug.
You can see a full Wonderbug story about at about 1:45 in this video, which involves people stealing secret technology that they can use to communicate with dolphins. The sheer genius of the Krofft Supershow was that they would have these insane plotlines, like science fiction crossed with an LSD-fueled jam session, and somehow make them work for little kids.
And honestly, Wonderbug was just one part of the madness, which featured Bigfoot as a main character. I love the scene at the beginning of this clip, where aliens are hypnotizing a bunch of humans trying to find Bigfoot.
You can watch a lot of the old Krofft Supershow on the Sid and Marty Krofft Myspace page.