The Ford Heritage plant (F-150s) at the old Rouge mill has tours through Henry Ford Museum. Don't buy Saturday tickets -- they are a sucker's bet as there is no vehicle production on most Saturdays.
I toured Flat Rock on Monday and missed seeing my car come off the line by about 90 minutes. I was offered the tour by a guy I do work with from AAI who is well-placed enough to get me in. I will never forget it. I have been to auto parts manufacturers (stampers, injection molding, etc.) and I've been to steel mills (1000 ways to die), but I have never been to a vehicle assembly plant and have never seen anything like this. It was fascinating. I was surprised by how relatively quiet it was, and you could practically eat off of the floor. The sheet metal welding looked to all be robotic which was pretty neat, but the coolest thing is when the engine, transmission and rear axle meet up and get fed up into the chassis. They are all sequenced with V-6's, V-8's and 4 bangers mixed intermittently with front or rear drive components depending on if it was a GT, V-6 Mustang or Mazda 6 chassis. Add to that that the Mustangs could also be coups or 'verts and the Mazda 6's come in, I think, 3 styles including a wagon, and it was American (and Japanese) ingenuity at its best. Truly amazing. I wish every car person had the chance to see it. And especially the Mustang people!