I would proceed slowly with the idea that an otherwise happy marriage isn't valid because of disagreement about a car purchase. A car is just a hunk of steel with some parts bolted to it.
That said, I think I get at what Matt's saying. My wife wants me to be happy. She may debate the wisdom of a purchase with me, but if she sees that it's important to me, she generally goes along with it, unless it's just bonkers to the budget, or for some other reason.