如果经常把你车子拦下，你属于”被拘留“状态。You are "detained" according to law. 警察让你下车，在几乎所有州，你都必须下车。If a police officer demands that you step out of your vehicle, yes, you have to get out, and you best do so immediately. No, the police officer does not need to tell you why. This true for anyone in the car, not just the driver.
#要求搜查你的车，打开后盖箱等等。警察会问“May I search your vehicle? " If you refuse, the officer cannot search your vehicle, unless he has a "probable cause"。 If he sees blood or a dead body in your vehicle, those are probably causes, and he does not need your consent to search it.
Normally an officer needs your consent or a probable cause to search your vehicle, but there are exceptions. One of the exception is the "plain view" doctrine. The plain view doctrine is that if an illegal is found in the view of the police officers without disturbing the exterior or the interior of a vehicle, that would not amount to a search needing probable cause.