Unless your long gun is a "Michigan Pistol" then it must be unloaded and separate from the ammo in the trunk or storage area. If it is a "Michigan Pistol" then it can be loaded, depending on interpretation of the law. Many people have successfully argued this, but I'm not willing to be a test case for it. Besides, none of my long guns were registered as such.
The Michigan pistol went away as of Jan 1, and I don't know if they were carried over once the silly law expired.