There is no need for a police officer to read somebody his Miranda rights before asking preliminary questions. If an officer sees drugs or a gun in a car after a traffic stop and asks something to the effect of, "Is that [marijuana / a gun]?", that's not going to rise to the level of a custodial interrogation.
Once a suspect is in custody, Miranda is only relevant to the subsequent use of self-incriminating statements. If the suspect makes no self-incriminating statements during an interrogation, even if his rights were not read to him in a timely manner there is nothing to suppress. If self-incriminating statements lead to a search, there may be grounds to to try to suppress the evidence found during the search, although there are theories that the prosecutor may assert (e.g., plain view, inevitable discovery) that can nonetheless result in the admission of the evidence.
When you have marijuana in plain view in your car, the officer has grounds to make an arrest. At this point, every police department in the nation has a procedure for searching a vehicle incident to arrest, and for performing inventory searches of vehicles that are impounded. It's highly unlikely that there are grounds to suppress the evidence found in the car -- but that's an analysis that must be made by a lawyer who has access to the police report and all of the known facts.

