A person's home is his castle, and it is accepted that in most legal situations short of protection of others or arrests no one may enter your home without your permission.
What about your car? In some states, such as New Mexico, your car is a legal extension of your home, and must be treated as such.
In most others, however, your fourth amendment rights are significantly limited while on public roadways, and if you are not careful you could end up forfeiting them.
While on the road and being questioned by law enforcement, you still have the right to refuse search unless probably cause is provided.
Probably cause is basically anything that gives the police officer a reasonable suspicion a crime has been committed.
Examples are drug-related odors, suspicious powders, a visible illegal weapon, or admission by someone in the car to a crime.
Absent probably cause, your car can not be searched unless you give permission.
Many law enforcement personnel will phrase a search without calling it such.
They may ask "You don't mind if I look around in the car, do you?" Do not be fooled by this.
You should always firmly state that you do not give permission for a search, and if you are not being detained you will be leaving.
According to "Speeders," 10% of searches result in arrests; this is the main reason why law enforcement personnel are anxious to search any car they can.
Should you find yourself under arrest after a search, you should contact a criminal defense lawyer.
Criminal defense attorneys are well versed in the legalities of searches and the fourth amendment, and will be able to look at your case and tell you if you have the possibility of filing for a dismissal based on law enforcement misconduct.
Do not fall victim to an illegal search; hire a lawyer as soon as you find yourself in trouble.