I struggle with this. Because the answer should be “most of them.” However, it seems like the majority of the ones that actually get enforced are for mods that are more common with minorities.
Jacked up truck? Not enforced
Squated Truck? Enforced
Lowered supercar? Not enforced
Scrapping Impala? Enforced
Loud Pipes? Not Enforced
Fart Cans and Popcorn? Enforced
I can keep going. I think it would be interesting to pack a car with 2500W of stereo and start blasting Toby Keith near a police station and see if you get a ticket, a warning, or an audience.
When you get down to it, American is thankfully home to many subcultures. These subcultures enrich us as a people. Many do car modifications, which is cool. But some of these modifications make the vehicle less safe, which is not cool.
To me, the big thing is if the person has changed their driving habits to offset the modifications. Take the stereotypical low rider that is driving slow. The lower speeds offset the suspension modifications. On the other hand, I see jacked up trucks hauling ass on twisty back roads where they driver is not slowing down to offset the lower limits of the vehicle. In this case, I have a problem with them.