How to convert tail light to LED?
You start with the typical voltage drop at the recommended current.



12V = 2.1V * 3LEDs + 20mA * R

Solve for R, R = 285 ohms

Then see how much current you get at the maximum voltage drop.

12V = 2.6V * 3LEDs + i * 285

Solve for i, i = 14.7 mA

This is probably OK, if it was much lower, you might want to reconsider and only put 2 LEDs in series. This is assuming that high brightness is important.

I haven't considered that your battery voltage is not a constant 12V. If it is 14V while the engine is running:

14V = 2.1V * 3LEDs + i * 285

Solve for i, i = 27.0 mA

This is still within the manufactures recommended LED current of 10 to 30 mA.

Note that this is only an example, you need to use the specifications for the LED that you have chosen.

And this is all hypothetical, as I said in my comment, I don't recommend altering your car.