Complex sentence: Most of the dealers in Westmont itself are owned by McGrath Auto Group and Autonation, where they deal in mainly high-end luxury brands such as Lamborghini, Audi, Acura, Lexus, BMW, Porsche and Mercedes.
Split into simpler sentences: Most of the dealers in Westmont itself are owned by McGrath Auto Group and Autonation. Westmont's dealerships are mostly high-end luxury brands, such as Lamborghini, Audi, Acura, Lexus, BMW, Porsche and Mercedes.