If you put it anywhere I would do it on the rear window in black so it's not as noticable, kind of like ghost flames or just do really small ones in silver on the tiny rear side window.
Maybe even some small ones behind the front wheel well.
Doing it on the roof you will see it but chances are you won't be able to tell what it is since it will be at almost eye level. It's going to look like you need to wipe something off of the roof.
just for kicks, write something on the roof using black electrical tape and see if you can tell what it says from a car length behind.
