Actually, you want to use a DC-to-DC converter that properly delivers the constant desired reduced voltage, rather than making a ridiculous stroboscopic light (aka PWM).
Constant current is more expensive and does not scale to a lot of LEDs or high power LEDs well. The scale wouldn’t be an issue with the tail lights, but the power kind of is. Take a look at modern taillights sometime, they all have heatsinks on them. I’m sure there are some; but I am not aware of any constant current taillights.