Never agree to let a cop search your car. Make them go through a formal arrest. If they search your car without your consent, and without a search warrant, they have to prove probable cause in order for anything they find or collect to be admissable in court. If you consent to a search, they no longer have the obligation to prove that they had probable cause.
Cops use "the system" to work to their advantage, why shouldn't you use your legal rights to protect yourself also?