Thinking about it, I believe the worst tactical error the North American automakers have made in the past three decades were neglecting passenger cars for SUVs and trucks.
Looking at the big picture, I can see that one error being a sort of "root of all evil" here. Because the major effort was being put forth with trucks and SUVs that continued to offer Americans the attributes they long associated with their cars before government regulation and oil shortages stepped in and somewhat sterilized them, passenger cars suffered that much more. And when the Japanese competition finally surpassed the Big Three in building a better car, GM, Ford, and Chrysler were left to play catch up. This subsequently led to billions of R&D dollars being spent to "reverse-engineer" Camrys, Civics, Corollas, and Accords. And that money would ultimately be wasted because not GM, nor Ford, nor Chrysler had the foresight to think two steps beyond what they had learned from their exercises in reverse engineering those cars, meaning that whenever GM, for example, would debut a new product, it was already dated in comparison to the new Honda product that debuted that year or the year prior.