Cops do NOT need a search warrant to search your vehicle! DO NOT believe that, because it's a false security blanket. Here's the deal.

1.) A cop can search your vehicle if they have sufficient cause. If they look through your window and see a bag, you're fucked. I don't think that smell is a sufficient reason, but I wouldn't take my chances on that.

2.) If a cop is dead-set on searching your vehicle, he can call and get a warrant and a K-9 unit.

3.) If you tell a cop that it's ok to search your car, they don't need a warrant or anything. Never tell them it's ok. Even if they threaten to get a warrant or call in a K-9 unit, you won't be in any more trouble if they do all of that, and there's a good chance that they're just bluffing. ALWAYS deny the consent to search. Even if you have nothing on you, don't allow them to search your vehicle because it will give you practice and experience in that situation.

4.) Avoid lying to them. While you're protected against self-incrimination by not being required to answer questions such as "do you have any drugs in your car?", that doesn't mean you can lie to the cop. If you can't be honest with them without incriminating yourself, then don't tell them anything. Use common sense when it comes to this, because you don't want to bring suspicion onto yourself by being silent, but you don't want to have an additional charge added if you're caught in a lie.

EDIT: Also, if a cop insists on searching your vehicle, tell them "no" but obviously don't physically stop them. If you can prove that your vehicle was searched illegally, then any evidence they have against you (ie. what they found in their search) can NOT be used against you in court, and your charges will most likely be dropped.