Due to the effects of heatstroke player character can go unconscious and while sitting in the drivers seat with the car running (not sure if relevant could be just passenger seat, happened to me when sitting in drivers seat with the car running on neutral when the character went uncon the car stopped running) the player won't "wake up" even if being resuscitated by another player.

Get heatstroke to the moment character starts going unconscious.

Sit in the drivers seat (or in passenger seat, like I said before not sure if the car must be running)