In a Horoscope, Fourth house is called Vahana Sthana(Position of vehicle). Venus is the planet of Pleasure, Luxury, so naturally Venus becomes the karaka (significator) for the Vehicle. Mars represents the structural aspect of the Vehicle as Mars would represent metal, engineering, machine etc. The function of the car or a Vehicle is to transport people, or conveyance. Transportation is the use of the car. Mercury is the planet associated with transportation, short distance travel, Communication etc. so Mercury also needs to be considered while buying or selling a car.
Benefic aspect on the 4th house, 4th lord, or Venus would be very helpful for the purchase of a car when the favorable main dasa or sub dasa is running, or even by a good planet transit over the 4th house. The association of the benefic planet could be by aspect or conjunction.
Please make sure these planets are in direct motion during the time you are buying a car. That is the 4th lord, Venus and Mercury. If those planets are afflicted, the structural and the functional aspect of the car would be seriously affected in the future. The Ascendant at the time of buying the car is also very important to consider. It should be free from afflictions as well. Who benefits from a sale whether the seller or the buyer etc can also be seen from the positioning of these planets involved at the time of Purchase.
For a good result, please make sure the Venus, Mercury and your 4th lord in the horoscope are well placed!