I've heard that if you wrap the Maglite in foam rubber, it doesn't leave a mark. 
On one of my 2xAA Maglites and my 3xD Maglite, I've added a Bust-a-Cap. Never had a chance to test it on a piece of auto glass, but it's supposed to be far more effective than just smacking the stock end cap against a car window.

On one of my 2xAA Maglites and my 3xD Maglite, I've added a Bust-a-Cap. Never had a chance to test it on a piece of auto glass, but it's supposed to be far more effective than just smacking the stock end cap against a car window.