First, your Miranda rights have nothing to do with searching a car or anything else. They have to do with giving a statement to the police that indicates guilt and if they questioned you without giving you those rights, then anything said to them must (or at least ought to be) suppressed by the Court. On the other hand, if you didn't own the vehicle and were just a passenger, you had no authority to allow them to search it and if the owner/driver (presumably) gave them permission, then he/she is more than likely responsible for anything found within it. However, there is a presumption that everyone in the car is responsible for the contents - unless one or more people take responsibility for the illegal contents.
Answered on Jan 18th, 2013 at 1:05 AM