Bird in hand.
I could envision a situation in which you come out ahead, sure. Say you buy now, as compared to a year from now, and the vehicle market shoots up in the meantime, while the stock market stays down (unlikely, given that the factors that will increase demand for vehicles are likely also the factors that would improve stock market performance, but whatever). What's the time value of keeping that money for another year? Well, if the market just sits around or even loses money, then you're better off getting a replacement vehicle now than paying a lot more for one next year.
The problem is that this decision involves precisely the same market-timing skillset as stock-picking does -- in fact, it requires more, because you need to accurately predict both the car market and the stock market. What happens if the stock market shoots up later this year? In that case, you're probably better off keeping your money in there for another year, even if you pay more for the car in a year for now. What if the car market stays depressed for two or three years? Then you jumped the gun and took your cash out of the market far too early and would have done better to wait another year or so and see how things played out. The problem is that you won't know what scenario this is until after the opportunity is over.
Also look at the magnitude of the pros/cons of each option. If you miss the trough on car prices, then maybe you spend an extra $1-2K when you do need to buy. OTOH, if you sell now, you could miss out on another decade of very low-cost performance from your current vehicle. The money you save by waiting to buy is very likely much higher than the extra cost you will save by acting now.
So, since you can't predict the future, and since the risk of waiting is pretty low, you should continue to follow the path that you know provides the best returns over the long run in most situations: don't spend money on depreciating assets before you need to.