where do you report the non registered vehicle parked in the street to?

There is a government subsidized house down the street from my brother's house.  The Mexicans who rent it have not only about 7 kids, and nobody appears to work, but they have at least eight cars, ATC's and jet skis on the front lawn (Stereotypical for Phoenix were he lives). All but two cars have no plates, and of course several are parked on the street, oil slicks beneath them and all.  My brother called the non-emergency line four times (complaining about loud music, unlicensed vehicles, high weeds, and litter in the street).  The police came and tagged all the cars because city code says that inoperable or unlicensed vehicles must be parked inside a fence or garage). Needless to say the place got cleaned up a bit.

The police. But, don't use 911. Find the right number.

To the police ..   How do you know it is not registered?

I would report it to the Minister of your local church.  He/she can pray for the person.

The police.                     

The local police.

Usually to the city, town, township, parrish, etc that you live in. 

To the public security department of your municipality. Some call that department bylaw enforcement.