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A more practical solution is to leave the windows slightly open so the hot air escapes.

There are places where a marginally open window will invite vandalism, or rain, or bugs, or smoke and bad smells.

ok, so in those places, you don't leave a window gap. in all the other places, you do.

Not practical if rain is forecast.

The car already has a rain sensor so it can close the windows automatically. I do believe some VWs already do/did that.

My car has that to activate the wipers. To say it works reliably is to lie.