As a general rule, police need a warrant to search your vehicle or home without your consent.
“Plain sight” is an exception to the warrant requirement. If an illegal object is in plain sight of the police officer, under certain circumstances, they can search your car without a warrant.
However, in New Mexico, even if something is in plain sight, if it is not dangerous, officers still have to go get a warrant, but they can seize the car temporarily while they go get a warrant, and then retrieve the item in plain sight.
That said, I would still advise not to consent to a search of your car.
When officers get a warrant, sometimes there are problems with the process that allow the car owner to challenge the legality of the search and the admissibility of any evidence against you that they may have discovered. For example, in the process of getting the warrant, they may have said something that was incorrect, or they misrepresented information.
It may be that they left things out they should have kept in, and you can challenge that warrant later on in court. But, if you consent to let the officer search your car, there is nothing to challenge later if you allowed it to happen, because you provided them what they needed without a warrant.