Spouse A wants to buy; Spouse B does not want to spend the money. Money can be the biggest deal breakers for marriage. Spending it when there is disagreement can build resentment and not spending it when it is necessary can create the same thing. I would suggest they talk about this in depth. The main questions to address are:
What is the need? Is the need real or imagined? Will purchasing a new car bring benefits or make their lives better? And, What are the financial consequences or benefits of spending the money? These questions should address both parties' interests and add clarity to their discussion.
Concerning the need, Spouse A would need to reveal the underlying interest in purchasing a new car. Spouse B would need to reveal the underlying fear of spending the money. All this can be very good, but will not work unless they truly listen to each other and consider each other's position on the subject. They need to keep their minds open to being educated about the need and the fear.
Maybe the most important question to ask is how they will feel six months from now if they don't buy a car and how they will feel if they do buy a new car.
Any other strategies you'd like to add, Readers?