At the request of a friend who wants this I had a look at modding my standard observation car marker pcb to see if it would work. The only flashing leds I have on hand are amber (for MARS lights) so thats what was used. Just replacing the rear led with the flasher results in all leds flashing together, which might suit some folks. But to make the markers stay on while the rear led flashes required only the simple addition of a dummy led. Yay! here's a video clip of the finished test car:
This pic shows the hookup detail for the flashing marker board in the premier obs car.
At the top you can see the amber 5mm flasher led poked thru the bezel opening after removing the OEM bezel. It’s soldered into the bottom of the board. To the lower left you can see the connector wires running from the marker board to pads on the led strip for power. So electrically the marker pcb is just like adding another section of 3 leds and a resistor to the led strip; nothing more.
The OEM red marker bezels extended way over to very near the center of the car. I just cut them off and the flat white leds are butted right up to the ends, but not glued. No need to glue them. Optionally you could remove the red bezels and drill the car body holes out to accept 3mm or 5mm red leds.
You can see both lower corners of the board have been trimmed with tin snips so as to clear the OEM lighting rack screws, for future removal if ever needed. The marker board can safely be trimmed anywhere below the wire connector pads. The black shrink sleeve in the middle encloses the added dummy led that flashes opposite from the amber led. This way you don’t see it flashing. If we wanted to have say 3 red markers flashing together we would simply omit the dummy led.
The board is hot glued to the end of the lighting rack. Easy peasy. The entire OEM light rack can still be removed for service if needed. The blue 500R pot is set about halfway, 250 ohms give or take. The side marker led wires are bent fairly flat to allow max clearance for the cardboard cover shown below.
This pic shows a simple cardboard enclosure in place to cover the board and leds. There is an extra section of WW strip leds mounted on it and connected to the main strip for power, just so the rear of the car is not dark.
The enclosure blocks stray white marker led light from flooding the back end of the car. We only want warm white in the passenger areas. The flashing rear led can be amber, red, white or any other color desired. All pretty straightforward. Another adaptation of the obs marker led board! 
