What would you estimate the era to be of the vehicles you are trying to identify? Prior to the 1930’s there were hundreds of independent auto manufacturers in the US alone. After that the big-3 either acquired or put most of those out of business by their economies of scale in production. If you are talking about very early cars, often referred to as “brass-era”, “veterans” or “horseless carriages”, you would probably best start by contacting the Horseless Carriage Club of America http://www.hcca.org/
They will be able to far more quickly help you identify obscure makes and models than anyone else as this is their specialty, or at least help lead you to automotive archives that specialize in that area.
D. Jordan Berson