Constructive Possession and Unwitting Possession
When law enforcement stops you in a vehicle you are presumed to have "dominion and control" over everything contained in the vehicle. This is a "rebuttable presumption" meaning the defense can put on evidence that the driver (or passenger) did not know the item/s were in the vehicle. This defense is called unwitting possession and can apply to anything found in the vehicle from stolen goods to firearms to drugs.
This defense is undermined when defendants answer law enforcement questions like "is there anything in the vehicle I should know about?" When you tell the Officer that there are unlawful items in the vehicle or, even worse, show them where the items are hidden it makes it very clear that you knew the items were there. I can't believe the number of cases I've read where the suspect shows the Officer where the drug, handgun, stolen credit card, etc is hidden between or under the seat.
There is a reason you have a right to remain silent and a right to an attorney. And you should use them. Absolutely never lie to an Officer as that can result in a charge of its own and possibly give an officer a right to arrest you and or search you vehicle when he couldn't before. It is difficult to do but the best response is to not answer the question or request an attorney. Please see my other guides for appropriate responses to police interrogation.
If you do have contraband in the vehicle it is actually excellent that the Officer has asked you to stop from the vehicle. Although you are more likely to be put under arrest (which was going to happen anyway, let's face it) the Officer has just undermined any argument that he needed to search the vehicle "incident to arrest" for Officer safety. Once you are safely handcuffed and in the back of the patrol car any search of your vehicle is likely unlawful. The Officer will likely need independent evidence that the vehicle is involved in criminal activity (like the smell of drugs or a positive K9 result) to obtain a search warrant. However, inventory searches of vehicles subject to impound may be an exception and this is not a well settled area of law.
As always, the best advice is to lawyer up early.