As someone said, you only need one requirement.. The one for the state in which the tow vehicle is registered.
So for Michigan. I visit the SOM - State of Michigan
web site, drill down to the Michigan Vehicle Code,and there is the requirement.. FOR ME.
Your state... IF not Michigan, may have alternative requirements.
Should you choose to tow 4-down (Which by the way many of use recommend and prefer) then be aware that a common theme here is "I'm towing a car, not a trailer, so those trailer laws do not apply (Has to do with aux-brakes usually)
Well, I did the research and in Michigan, a car in tow *IS* a Trailer under the law. so brakes,,, Needed