Been running a 3rd flashing light under the license plate for 4 years now...
It's LED and I got the 3rd brake light kit from Fleabay for about 8 bucks...
It's a bit small unit and the wiring is very thin but it works.
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I had an LED light stick over my license tag for 3 yrs, but being naught but sino-plastic, the AZ sun cooked it to disintegration, so I found an original Mopar license tag fixture for the '66, (ePay) cleaned it up and found an LED bulb for that socket. That proved to be so bright that I hooded the top half of the clear acrylic lens with aluminum foil tape to keep the light on the plate and NOT in the eyes of traffic behind me. These fixtures make it very difficult to use a locking gas cap, so I removed my cheap VatoZone one and restored the old simple steel factory cap. Since I HATE locking gas caps, I count this as no major loss unless I go somewhere that has a high probability of siphoning theft. I can take a pic of it come sundown if anyone is curious.
Ugly as an LED 3rd light bar is, after being rear-ended over 4 yrs ago, which enabled us to purchase Mathilda, I mean to make it GLARINGLY OBVIOUS when braking. A C body Mopar with a ruined rear end is uglier than almost any light fixture.
Eventually, I hope to find a tasteful small amber side-marker for the front turn signals and fog lights.