In the 1984 case United States v. Johns, the U.S. Supreme Court ruled that police officers do not need a warrant to search packages in vehicles if they have probable cause to believe those packages contain illegal substances. The decision came after three defendants were convicted of marijuana possession and intent to distribute based on evidence found in their trucks without a warrant during two separate incidents by Arizona law enforcement officials. The defendants argued that this violated their Fourth Amendment rights against unreasonable searches and seizures, but the court disagreed with them by a vote of 7-2, stating that motor vehicles are inherently mobile and therefore subject to different rules than homes or offices when it comes to searches.

In the dissenting opinion for United States v. Johns, Justice Brennan disagreed with the majority's decision to allow warrantless searches of packages in vehicles based on probable cause alone. He argued that this ruling significantly erodes Fourth Amendment protections against unreasonable search and seizure by extending the automobile exception too far. According to him, while it is reasonable to conduct a warrantless search of an automobile due to its mobility and reduced expectation of privacy, these reasons do not apply equally well when considering closed packages within a vehicle which have higher expectations of privacy. Therefore, he believed that law enforcement should be required to obtain a separate warrant before searching such containers even if they are found inside an already searched vehicle.