1)Get a ham license. Every state law that I know of allows
hams to have radio that will recieve police frequencies
in there car. However not all LEO will understand. The
KY AG wrote a lettr for hams to carry in KY to help avoid
this hassle. I supect the ARRL or the rec.radio.scanner
group can provide specifics on a state by state case.
2)Be prepared for some interesting noise problems. Modern
engines/brakes/suspenesion often have rather noisy CPU
control systems that produce odd noises in the 5~30MHz
frequency range.
3)The ignition system in modern engines is a killer noise source.
Your radio will need a very good noise blanker.
4)It is very difficult to get a really effective SW antenna on a car or
truck.
An active antenna will work, but tends to magnify the mentined
noise
sources.
Terry