Tell her if she wants the car to go back to work and pay for it herself.

Why should you place yourself in a Father position that doles out the Yes's and No's? It can only build resentment in her.

For every harebrained item she wants to buy, she will need a better job. Insist that she pay half of the household responsibilities, as well.

Take yourself out of the position of BB's financial advisor, as much as you like it. (come on, you MUST like it, you've been doing it for many years!)