I’m guessing it’s a rental car the family rented 5 weeks after she disappeared. The police say that they found DNA matching the girl IN the trunk.
The suggestion has apparently been made by the parents that they had put some of the girl’s things in the trunk, which could have left DNA, and the police are now saying that somehow their DNA test could tell the difference between DNA which was transfered from the body to a teddy bear to the trunk, and DNA that was transfered from the body to the trunk.
I have no idea whether it is possible to distinguish between the two, although I would imagine if a body was put in the trunk there would be more dna lying around than if it came from a secondary source.
I believe that scientists can tell whether blood samples came from a live person or a dead one. If there were blood samples from Madeleine in the rental car, and the samples were from a dead person, then the her-sister-wore-her-old-shirt defense won’t cut it.