Originally posted by WyrreJ
How about using one of those as a replacement for the top brake-light. Instead of just flashing like Tone's mod it could actually tell the people, "I am stopping, you better slow down too!"
Huh? There's a switch for that already. If you press on the brake pedal very lightly you can activate the brake light without engaging the brake systems. Cars are designed that way for the very same reason you mentioned.

Getting back on topic... if you go to arts and crafts stores you can probably find a adhesive stencil film (photo frisket). With a really sharp blade and steady hand, you might be able to make a nice stencil to mask off the symbol you want... however, it's probably best if you keep it as simple as possible.