I know MS has no scanner law, so we're fortunate to be free to mount what ever the hell we want in our vehicles. State law considers your vehicle as part of your estate. So you can have scanners, guns, all sorts of things legally in your car.
However for some place like Florida it's illegal to have a scanner in your vehicle (IF) it is connected to the vehicle's power, and or an antenna mounted on the vehicle. They consider that an installed scanner. They don't seem to have problems with portable scanners which operate under their own power and antenna.
Where I think the exception comes is from amateur radio equipment, which 99% of has the ability to receive frequencies in the public safety and government bands. Since there are so many radios out there that do, and the FCC type accepted them, they had no choice but to offer a provision in the national scanner laws to allow licensed amateur operators the ability to have such equipment installed in vehicles.
This has nothing to do with scanners. It's for FCC type accepted amateur radio equipment.
You can drive through Florida with your Yaesu FT-8900R mobile which will cover from 29mhz to 900mhz and it does scan memory channels and bands, but it's a ham radio transciever. If you're licensed you're ok. If you have a BC-796 or or other type of scanner mounted then you're likely to have that scanner taken away from you if stopped.
more info on this can be read here:
http://www.afn.org/~afn09444/scanlaws/
The specific question answered here:
http://www.afn.org/~afn09444/scanlaws/federal4.html#pr9136
BTW, AWESOME online Practice exams here:
http://www.eham.net/exams/