Ford had the Ranger beginning for 84 IIRC, and Mazda did not adopt it until several years later, having their own truck that they started for 82 IIRC. the Mazda truck prior to that was rebadged for Ford(Courier).
The original Ranger was the basis for the Bronco II and the original Explorer, even using the same body panels and nose, etc. This was still before mazda used it. To this day, the Ranger is the same one as they made in 84. Even after Mazda began using it, it still had the same body panels as before. I think even now the doors and roof and pillars are the same as the 84. Mazda sold a rebadged Explorer called Navajo, but you Americans would know that. It wasn't sold up here.
Anyway, I'm just saying that the reason I was thinking the B series is just a rebadged Ranger is because Ranger existed a few years before Mazda adopted it, and it's still the same truck now, and one would assume it is still made in the same plant as ever, which would have to be a Ford plant. The Escort and Protege/323 were made together on a Ford line in Wayne, MI., even though they shared no body panels. What plant is the Ranger made in? Is it a Mazda plant? I'm not saying you're wrong, I have no idea. I'm saying if I had no other info this is what I would guess.