In addition to the major concerns about CO there is also the minor combustion product condensation issue.

Once upon a time I set a radiant propane heater to running in the rear uninsulated camping box of my unimog while I was hanging out by the fire talking with the guys.

While the box was warmer than it would have been, there was an enormous amount of condensation from the combustion water vapor running down the walls of the box. The heater was shutoff for the night and there was plenty of ventilation due to seal leakage for sleeping. But the humidity in the box was really high - not the best for comfort.

In this case you may wind up with the headliner of the car being softened and coming down from the ceiling over time. Or you'll just get a huge case of inboard rust in the interior.

A minor consideration compared to being dead. But yet another reason this isn't such a good idea.