Heat stroke is agonizing. You are aware of every moment you are dying practically. Its not excruciating in a 'limb has been chopped off' kind of way but in a 'you are fully conscious as your cells begin dying' kind of way. Sure you eventually will lose consciousness but every single moment up to that will be utter hell. Hell depending on how you plan to 'be unconscious' in your car before the heat gets too bad you could still wake up right in the middle of things as you are biologically wired to avoid dying and your body will likely, if possible, try to rouse you out of the death trap you are in and you will awake to unimaginable hell. I would not wish that death on anyone.
Sincerely,
Someone who nearly died of heat stroke when they did not wish to die.