I believe the bulbs flash in certain locations behind the people and buildings behind backglass. It's not a mod just blinking bulbs.

They're not flashers but rather blinking bulbs. They originally came from the factory with them installed. As a matter of fact, a lot of Lawlor's games have them. ES, WW, TZ, TAF, TAFG, NGG. The incandescent #455 blinking bulb used in ES has been discontinued and prices have soared recently. You could use an LED equivalent.

I recommend using LED blinkers if the rest of your backbox is going to be LED. Otherwise I found that my LED backbox lights pulsed to the frequency of the incandescent blinkers.