Okay, for the sake of maybe helping you, I'll assume you're serious.
Something like 1 in 25 suicide attempts fail. Heck, I've tried to take my life twice, with prep, and failed. I was just lucky that on both occasions my survival came with no long-term consequences. I know you're in a lot of pain. And it must be an excruciating pain for you to think you deserve to go this way. But you don't. You're clearly articulate and you clearly have a lot of spirit, as I gather from your heated replies.
If you survive, you may not get a chance to attempt again. It's really as simple as that.
It might be because you lack the mobility/mental capacity to carry out a second attempt and it might be because the watch you'd be placed under would be out of this world because this would be taken ferociously seriously, no matter where in the world you are. it's a method that doesn't just endanger your life, but the lives of everyone around that burning car.
People who hang themselves? Biggest risk is they fall and injure a person who's taking down their corpse. People who take poison can make sure that no one else gets hurt by leaving detailed notes describing what they've taken and how anyone that discovers them can remain safe. People who shoot themselves...well, they're only able to fire one bullet, and they can make sure there's no one behind the wall they're nearest.
What you're proposing is a death trap.
It doesn't matter where you do it. Busy city or countryside...fire spreads. People are curious.
Fuck, a bunch of my acquaintances went to the middle of nowhere to take drugs a few days ago, literally drove for five hours on impulse after checking Google maps and picking a place at random. Things like that totally put me off an attempt in nature.
The world is full of people who are hiding and full of people who are seeking and setting fire to a car will catch attention. A lot of people are having parties/meeting lovers in nature due to covid, and I'm sure this isn't a phenomenon exclusive to the UK. I know a couple who camped together all summer because their housemates wouldn't let them meet in their respective houses.
You could be responsible for the deaths of others.
If the violence of the method appeals for you, there's an entire wealth of resources on violent methods.
The one thing we can do, even in our suicidality, is protect and love others. Above all the risks, the risk of getting it wrong, the risk of getting it right and regretting it- the risk of hurting others should leap out to you the most.