Complex sentence: Whereas the dealer focused promotionals were previously the most important business, by the mid- 1960s a whole new market involving customizing and customizers, often linked to TV and Movie themes was driving AMT, and other model companies.
Split into simpler sentences: Whereas the dealer oriented promotionals were previously the lead business-by the mid- 1960s a whole new market was tapped involving customizing and customizers. These were often linked to TV and Movie themes which now drove AMT, and other model companies.