Before you ask for a refund, make sure that you will get the full amount back.
When I made my deposit, I read the fine print on the sales confirmation slip. It has a line that says you are entitled to a full refund of the deposit provided that the car has not been registered to your name. If they have started procedures or have already registered the car then there are penalties. Just can't recall how much penalty.

Try talking to the GM and sales manager with a lawyer or DTI rep.
Explain that you paid the down and they accepted your money within the promo period. A contract is defined as a meeting of the minds between 2 parties. For your case, they offered to sell a car with free insurance, you accepted and paid the DP. The definition of a contract has been fullfied. The fact that they did not have a car available is beside the point. It is their problem, not yours. You do not have any contol over it. Since you paid the dp, then I assume that you or the bank will pay the balance upon release of the car.

If they still refuse to give you the free insurance, ask for your money back. And tell them to stick it where the sun don't shine. Tell them to watch you buy from their competitor.