You could look at objective car sales data, but that tells you what is selling, not what might sell. I’d suggest a survey that is sent to an audience who has indicated interest in an upcoming car purchase or who frequents car purchase websites. You could propose a few alternative car models, list the features, and ask what they would be willing to pay. If the numbers give room for profit, you’ve got initial validation. You’ll want to work with a professional researcher who can ensure you are surveying the right people, asking in an unbiased manner, statistically analyzing, and inferring results appropriately.