Demo vid. I know this is a bit brief and more of an idea then an instruction, but it'll sure freak people out seen at night.
More of an idea then a hardcore instruction. Playing idly with stuff on my desk, I noticed these look a lot like a police red/blue light, something I once freaked out over when my son was playing with a red/green/blue flashing ring with the green broken in the back seat. Seeing it out of the corner of your eye has a near-visceral reaction, especially when driving.
Step 1: Supplies
Two LEDs, one blue, one red flasher.
Battery (here a 2032, but any 3v or two stacked 1.5v ought to do)
Magnet (optional, if intended for metal attachment)