U.S. v. Zamora-Garcia - consent to search vehicle sufficient to allow search that turned up drugs
This is a simple example of why it is important to know, and exercise, your constitutional rights. The defendant here was pulled over simply because something under his vehicle was dragging the ground. He appeared nervous when the officer was talking to him, so eventually the officer asked for consent to search the vehicle. All this guy had to do was say no (thanks to the Fourth Amendment), but he didn't. He consented, and a preliminary search of the vehicle turned up a welded compartment attached to the underside of the vehicle. The vehicle was removed to State Police headquarters and eventually drilled and opened, and lots of meth was found.
Had the guy simply said no to the search the officer wouldn't have had probable cause to keep him any longer and he would have been on his way. If you are asked for permission to search your vehicle, you have the right to say no. If the officer claims probable cause to search anyway do not interfer and instead take it up with a qualified defense attorney to see if you have an argument later.