A car in particular is a reflection of a person’s self-image and possibly sexuality and indeed any part of a car has significance.
The back tyres might suggest our support system, the steering wheel the way we control our lives and so on.
If the brakes are not working we are not exercising proper control over our lives. Too many people in the car would suggest that we feel overloaded by responsibility. Also consult the entries for car, carriage / cart / chariot, passenger and wheel for additional information.
What Does Car / Carriage / Cart / Chariot Mean In Dream?