What's the simplest way to dim LEDs for tail lights?
A standard 1157 dual-filament tail light bulb has a lumen ratio between brake/turn signal and running light filaments of about 10:1 (402:38 lumens). 

Since the forward voltage of the LED won't change much and the vehicle electrical system voltage is fixed, and since LEDs are fairly linear lumens per ampere, you can simply use resistors in about a 10:1 ratio (actually a bit more since you're using a 6V electrical system and the LED voltage will increase a bit at 10x the current). 



Another way would be to use multiple LEDs and power only some of them, at the same or at reduced current. 

Taking a typical power COB LED datasheet, forward voltage is 2.25V typical at 350mA, so if you decide to run it at 250mA/25mA you'd use resistors of about 20 ohms/200 ohms. You can't compare lumens directly because the LEDs are more directional than the incandescent bulbs and the filter absorbs a lot of the light (everything that isn't red). Note: I've assumed the '6V' vehicle electrical system runs at about 7V with the engine operating. 

I suggest picking the low value resistor to give the maximum current you want (if the brightness isn't good enough, add LEDs/resistors or use better LEDs) and pick the higher value to reduce the run light brightness to the correct value. The higher value resistors will be small and cheap.