What to do if a specific label of a category appears only a few times?
Since a few cars models make your 20% data, you can create a similarity matrix of all the car classes with these 2-3 car models. So in this matrix, each car model has 2-3 values depicting its similarity with those 2-3 car models. You can now add these new features to your existing features to get some improvement over the classification of the underrepresented car models.

You may play with the number of cars models to compare each car model to. Here, these 2-3 car models cover 20% of your data. It might be that 30 car models would cover say 40% of the data which is still useful as there are 3000 car models.