Search of vehicle vs. search of residence

Details: A senior federal agent is assigned to coordinate a particular investigation in a local high-crime area infested with drug dealing.  A possible "stash" house  has been identified and there reason to believe drugs are being sold to between 30-40 different customers each day.  Thus far it has proved impossible to introduce an undercover agent/officer to the dealer to make a buy.  In addition, it has been determined that even if an agent could get inside it would be too dangerous to do so given this gangs predilection for violence.  The chosen strategy is to set up surveillance on the location and observe the constant flow of persons in and out of the location.  After several weeks of surveillance, it has been observed that approximately 30-40 forty vehicles drive to the location each night between the hours of 10:00pm and 5:00am.  They park and the driver or passenger runs inside. No one stays for loner than three minutes.  

Based upon drug enforcement  training and experience it is known that this pattern is consistent with substantial drug sale activity.  The agent decides that the best way to proceed with the investigation is to attempt to obtain a search warrant for the residence.  He will establish that the individuals driving to the residence are receiving narcotics then the warrant can be obtained.

The agent set up surveillance that evening with the plan that when cars leave they will be stopped and searched.  If narcotics are found (even better if some of the customers decide to cooperate and give statements) he can apply for a warrant for the residence.

Discuss the requirements for searching a vehicle as opposed to searching a residence.  Is the threshold level for probable cause the same?  Do both require the issuance of a warrant?  Begin with a brief review of the constitutional provisions controlling searches and then distinguish the two scenarios.