I think the CO level would have been pretty close to nil. On the windward side, I could feel a little bit of air being pushed in past the door seals (old car, you know). No doubt a bit was being pushed out on the lee side. Adequate ventilation for four tiny candles IMO. (I don't think my house CO detectors work below 0C.)

I don't believe sun was a factor this time, though it would be in other situations. It was late in the day, deep overcast, snow blowing horizontally across the fields. The car had 3-4 inches of powder snow across the top, including the windshield, hood, etc.

Not a scientific test by any means, but instructive none the less.