Something I remember from my CB days was that a lot of people would rig illegal amplifiers with too much power in their cars or trucks. The possibility of getting caught was pretty low, and they even sold the radios in a lot of truck stops. I think they still do in many places.
You could sometimes tell when you drove past someone with an amp because your car radio would start making all kinds of funny noises from the interference. I guess that's why you weren't supposed to have the amplifier in the first place.