Complex sentence: The plummenting sales of the larger Mustangs led Ford to return its ponycar to its roots beginning with a downsized Mustang in 1974-being based upon the Pinto platform this was not well received by enthusiasts but fit well into its intended market.
Split into simpler sentences: The plummenting sales of the larger Mustangs led Ford to return its ponycar to its roots beginning with a downsized Mustang in 1974, based upon the Pinto platform. The car was not well received by enthusiasts, but it fit well into its intended market.