Your problem is that you are not representing the relationship between a car and it's model directly. Don't relate a car to its manufacturer directly, relate via the model.
Implement your data model this way:
In your Entity Framework model you can now easily see the make and model of any car. In assigning the model to a new car object, you can drill down from a selected make to the available models.
If this is a robust application with historical data, you'd probably have to include year
in the mix, such that models only exist for certain years.