A resistor can certainly work to drop the voltage for the LEDs, BUT presently many of them run on a lot more than the 20 mA of years gone by. I have a bunch of strings of very bright white LEDs that run on about 3.2 volts and on 80 to 100mA. They will give off some light at 20mA but not nearly what they deliver with more current.

Ah, well, the spec sheet says 20mA ... and these are mood lights... NOT designed for bright light... useful for a geezer wandering around his trailer at night... kind of like a night light.

 

I don't see it mentioned before, but what you can do is wire 3 of your lights in series and run the total off the 12V supply.
No extra resistors needed, just some wire.

 

I don't see it mentioned before, but what you can do is wire 3 of your lights in series and run the total off the 12V supply.
No extra resistors needed, just some wire.

Certainly putting the LED devices in series is a good way to improve efficiency, but you still need to have an effective series resistance to limit the current, since the LED current to voltage relationship is so very non-linear. But with a single resistor the power wasted will be much less. So the series setup with a smaller series resistor is the best simple and cheap way to go.