The police can search your vehicle in a few limited circumstances. First, the police can search your vehicle any time you give the police consent to search. Second, the police can search your vehicle any time the police have probable cause to believe that your vehicle contains contraband. Third, the police can search your vehicle incident to an arrest. This third scenario is often abused by law enforcement when law enforcement is looking for a legal reason to search your vehicle. Many people don't realize the police can arrest for any violation committed in their presence, including traffic offenses. It is a common practice for the police to follow an individual that the police believe is "suspicious" until that individual commits a traffic violation. The police will then arrest the individual for the traffic violation and the police can then legally search the vehicle incident to the arrest. As unfair as this sounds, it is legal, and is done everyday.
If you or a loved one have been arrested it is wise to consult with an experienced DFW attorney before talking with law enforcement.