You logic is flawed. OK. America made the assembly line. But you are saying that just because Ford used the assembly line to make the cars cheaper, America "invented" it. Unfortunately, it doesn't work that way. Virtually every industry uses the assembly line, so every product on the market would have been invented by America right?
Like I said, mass marketing something is not the same thing as inventing it. That is the same thing with light bulbs. An englishman called John Swan had already patented the bulb one year ahead of edison in england. Edison took the design, improved it, mass marketed it, and all of a sudden he's the inventor!!! Unfortunately some things in history cannot be corrected.
edit: By the "modern automobile" it's obviously not a steam engined but with an internal combustion engine.
"Internal combustion engine automobiles were first produced in Germany by Karl Benz in 1885-1886, and Gottlieb Daimler between 1886-1889."
http://en.wikipedia.org/wiki/Automobile