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)
Step 2: Build
Attach both LEDs as per http://www.instructables.com/id/LED-Throwies/
to the battery, tape up with magnet. The blue LED will glow, but when the red blinker blinks it will temporarily overpower it. This results in a blue glow interrupted by red flashes. It won't exactly look like The Man showing up under close scrutiny, but from a distance seen briefly it's very powerful.
Step 3: Done
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.