Consent to search car

First, you need to talk to a criminal defense attorney if you have not done so already.  There may be facts associated with the arrest that you are not including in your question but that might improve your legal situation.  To answer your question:  No, the police do not need to inform you of your right to refuse the search of your car. 

Some general information.  If the police officer who pulls you over does not have a warrant when he pulls you over (why would he?) or some other legal authorization to search your car, then he cannot search your vehicle.  If he does not have the legally required probable cause for the search, then any incriminating items he finds during an unauthorized search will likely be thrown out by a court, or suppressed.  In some cases, if the officer has a reasonable suspicion that some criminal activity is going on, that can justify a search of the car.  However, the Supreme Judicial Court of Massachusetts recently held that, by itself, the odor of burnt marijuana cannot justify an exit order and search of a vehicle. 

Unfortunately, if you give a police officer permission to search, you essentially toss your constitutional protections against unreasonable searches out the car window.  The lesson here is that it is OK and perfectly legal to politely decline when a police officer asks you if he can search your vehicle, even if the officer seems determined to do the search regardless of how you respond.