In Hainsberry's play, cars are associated with Walter's job. As he works as a driver, which is a position of service and therefore subservience, the car can be seen to represent power, control, or dominance (especially material/financial dominance).
We might also say that the car represents the economic system that exploits the Younger family. Though they work in the system - even driving it - they do not have any stake of ownership in that system. In this case, the car symbolizes disenfranchisement.
For Arthur Miller's play, cars can also be seen as symbols relating to power. In the car is a tool used to demonstrate Willy's decline. First, his decline is mainly professional. He cannot drive to the area where he is supposed to be doing his sales. He blames his failure on the car.
Later, he kills himself in a car, proving his moral and psychological failure and ultimate decline. We might say that the automobile is the vehicle of Willy's undoing, speaking both literally and metaphorically.
After all, Willy loses his place in the company when he fails to change with the times. He does not recognize the need to change and instead insists on mantras of old, repeating the idea that a person must be liked to get ahead. Personal charm will not help Willy drive to New England, yet personal charm is what he continues to harp on when Biff arrives back home at the opening of the play.