Originally Posted by Lima Tango
Typically, an LED will need a current limiting resistor in series with the LED. The value of this resistor will depend on the voltage of the source, the forward drop across the LED, and the LED's operating current. This is a generic answer; there may be some LEDs that have this resistor already integrated with the LED. Especially if the LED is packaged up for a specific application, for example to replace an existing incandescent bulb. You need to have information for the specific part you're using to avoid a mishap.
You can also buy a $15 blinker relay specifically for leds. I got mine at cycle gear and also online. PLugs right into the harness where the old one was and you don't need to mess with anything.