Hello. My name is Marc. I'm a licensed attorney and I will be happy to answer your question.
My answer might disappoint you, I'm afraid. Whether your car is commercial or non-commercial does not matter. The fact is that MA law (and most states) prohibits placement of any obstruction in the front or rear windshield.
Now, you might have a very small LED light that you want to use. And you might reasonably believe that it's too small to obstruct your view. If that's the case, then it would be legal. But you must be careful because MA law doesn't state any precise measurements or dimensions that specifically describe what is legal and what is illegal. That means that it would be totally up to a police officer to decide whether you are in violation of the law. For example, if your sign is only a few inches high and, in your opinion, it does not obstruct your view, a police officer might still pull you over and give you a ticket. The fine for this particular violation would be $250. Your alternative would be to go to court and hope that the judge agrees with you that your sign is not obstructive.
So, in sum: Your sign would be illegal if it obstructs your view. But even if it doesn't obstruct your view, a police officer might still give you a ticket. Accordingly, I would suggest that you put the sign in one of the side windows instead. There's no law against that.
I hope my answer has helped you to better understand your issues and options. If so, please be sure to rate my answer, since that is the only way I can receive credit.