http://public.findlaw.com/abaflg/flg-11-4b-3.html
Can the police legitimately search my vehicle without a warrant?
That depends on the circumstances. A key factor is whether you've been arrested. For example, the police usually would not have the right to search your automobile when you are stopped only for a minor traffic offense such as speeding. However, if the violation requires that you be taken into custody (for example, a driving under the influence (DUI) arrest or driving with a suspended license), the search generally would be permitted.
Even when an arrest is not involved, the police have more latitude to search a vehicle than to search a home. The U.S. Supreme Court recognizes an automobile exception to the Fourth Amendment's protection against warrantless searches. The rationale for permitting warrantless searches of cars is that the mobility of automobiles would allow drivers to escape with incriminating evidence in the time it would take police to secure a search warrant. The Court has held that a person expects less privacy in an automobile than at home.
No one ever said, "A man's Chevy is his castle."
http://public.findlaw.com/abaflg/flg-11-4b-1.html
Suppose a police officer wants to search my car?
Ask why the officer wants to conduct a search. If you have absolutely nothing to hide, you could save time and effort if you simply let the search proceed. If you don't want the search to proceed, you should state clearly that you do not consent. Denying a search is not an admission of guilt. Ask courteously whether the officer has a search warrant or if you are under arrest. If the officer replies that you are under arrest, ask for an explanation.
Usually, the officer is permitted to conduct the search only if
# you consent; or
# the officer has probable cause to believe that the vehicle contains incriminating evidence; or
# the officer reasonably believes that he must search the vehicle for his or her own protection.
http://public.findlaw.com/abaflg/flg-11-4b-2.html
What if a police officer insists on searching my car?
Don't interfere. You can always challenge the legitimacy of the search later in court.