Lowest Price Guaranteed! The Diamond Auto Group has been in business for over 65 years as a family owned family operated business. We are now New England's largest Cadillac, Chevrolet, Buick, and GMC dealership and we are growing even bigger!We want you to bring us your best deal, and we will show you how will not be under sold, or out traded! We want to earn your business today! Note: If your looking at one of our New Car Truck Internet Purchase Price Specials, we have include all Ally and GM incentives to the incredibly low price that we have posted online.