Depending on what state you are in, there are various laws against having sex in public which includes having sex in your car. Some states may be more severe than others, but in general, the law is the same throughout the USA with regards to having sex in your car in a public place. The main law this falls under is public indecency. One can also be charged for disorderly conduct and if the engine is left on, public endangerment.
If you are caught by the authorities as a first offender, then you can expect the minimum sentence. This usually involves a short probation period. It would be classified as a class one misdemeanor. The maximum time a person can usually be charged for as a first offense is two years imprisonment.
In the case where a person is charged a second time with public indecency, a harsher sentence will be passed. The range of the sentence is anywhere from a a probation period to close to three years imprisonment. The severity of the offense is based on the actions of the accused. If the act took place in the middle of many people then it would be more severe than if the street the car was parked on was deserted.
Third time offenders are looking at a maximum of almost six years imprisonment. On the third offense, the crime is considered a class six felony and is therefore more severely dealt with. It is important to remember that at any time, if one of the people involved is underage, it is immediately a felony.
In many states, it is required that people who are caught displaying lascivious and lewd acts of nudity in public must be registered as a sex offender. It is strongly advised that you avoid having sex in your car if there is any possibility that someone will see you. By being a registered sex offender, many of your rights will be taken away from you such as the availability of loans as well as access to certain public areas.