I found the Mazda tour very interesting. They had, when I went, approximately 4 tours each day in Japanese and one in English, and explained both how the production line works and their theory of operations, how and why they do things the way they do, as well as showing the history of the company and some of their cars from the past history of the company and so on.
The hardest part was finding the factory, the suburb of Hiroshima that it is in is off the usual tourist path so signs are not in English, but a bit of wandering away from the railway station perpendicular to the railway tracks led us there. Once you get closer it's difficult to miss