Well, you can try these from Amazon, and install just one for starters, in series with the turn signal line to both rear fixtures for starters, and see how much it dims the LED's down. 6 Ohms should not make all that much difference in the intensity, but it will lower the current through the LED's, and every bit helps to prolong their life.
Amazon Items
Ideally, it would be better to have a resistor break-out box once can use to arrive at the right resistance value for intensity of the LED's, but most people don't have such a thing, and may not know what shops in the area have one. THEN you could order a different resistance value for the running and brake/blink LED lines. Some are as high as 30 ohms. I have seen LED tail lights that are WAY too bright.
Anyway, I would say that for most LED fixtures on vehicles, the 6 Ohm resistors should not post any problems. If you really want to experiment, install the resistors only on one side, and see what difference they make in the lifespan for yourself...if you feel adventurous. You can always remove the resistor if you don't like the results, and re-splice the wiring.
If you don't know to do that, then let someone do it who knows how to crimp splices into wiring. Soldering is even better, but, again, most people don't know how to do that kind of stuff.
Those resistors are also ideal for converting incandescent tail lights to LED. The problem is that you don't gain any efficiencies in your electrical system in those conversions because the current draw is about the same with either type of bulb when you connect the resistor in parallel with the tail light fixture. That's how they keep the hyper-blinking that generally happens when replacing incandescents with LED's in vehicles.
Do this ar your own risk, of course. Wouldn't want you to get shocked...
MM