I am in need of a new car now as mine is on a quick decline to repair bill-hell, I could cash in my BTC now and buy something pretty reasonable, about £8,000. But I'm not sure this is wise as I think we still have a long way to go with the price.
This will sound like your dad, but anyway...
I want you to think of this another way. You should separate out the issues. A basic need in life for where most of us live is a healthy reliable car [1]. If your car is on the way out or causing stress in some manner you need to acquire a new (to you) one.
Never go into debt to buy a car for more then 6,000 USD. If you can't afford a 6k car then get a loan to acquire one worth about 6k. Having something with basic reliability, clean, and well running is important to freeing up your life to pursue other ventures without needless wasting resources.
I would sell 6k worth of btc to acquire a car today if you can't pay cash for it. This is coming from a guy that has sold btc in the past to acquire things that if I hadn't the present value of btc lost for those items is amazing. A 300k motorcycle (purchased for 4k usd worth btc at the time), the 60k necklace for my wife (500 usd worth of btc at the time), etc.
I don't regret any of this.
I would NOT take out a conventional loan for a 6k car if I could acquire it with selling btc today, only because you're doubling down on your finical risk.
Move your life forward a little, and if this crashes you are in a better spot then before. I don't believe it will crash soon (maybe not ever), but I do know that regrets come from both sides of this game. If you can modestly and prudently move your life forward by liquidating a few btc then do it.
I think a good rule is to pull 20% of your btc profits annually for things like other investments, or a little better standard of living.
Try not to make it an all or nothing game.
[1] - note I didn't say flashy/expensive high end fools toy