Word has it from Neal Kublan that the Spider Car was Stan Lee's idea, Kublan rejected it. As he recalled, "when you tell Stan Lee you don't want to make a Spider Car, he takes it personally."
Lucky for us Mego relented and the Spider Car sold quite well, Marvel gave it a cameo in the comics (and mentioned Peter Parker couldn't drive) and history is born.
Thanks to the folks at susamy.com for the nice photos of the loose car! Box photos are courtesy of me.