No, you don't. In fact, most people think that they have to, because they've been stopped and they're somewhat intimidated and they do give consent. Just like your home, a law enforcement officer needs a warrant to search your vehicle, so you don't have to consent. There are limited circumstances where they can search if they see a weapon that they think you can grab or if they think that there are some type of criminal evidence inside the vehicle. Typically, you just say, "No," and tell them to go get a warrant.