Just one thing to mention about LEDs, and this probably won't matter to 99% of the people here. Big truck drivers are experiencing a minor problem with LEDS. At first they replaced every light on their trailers with them. But for those up in the north it presented a huge safety problem. It seems that since the LED is so much more effiecent it does not heat the lense up enough to remove snow, ice, etc. So their marker lights, which must be visible by law, where getting frozen over. So now what they are doing is replacing/adding all sorts of LEDS, but keeping their marker lights regular bulbs.
As for adding the resistors to make the flashers work, that is almost defeating the purpose of using the LEDs in the first place. You would be better off searching out a controller to flash the LEDs without the resistor in place. With the resistor you still get a very long life bulb, that is super shock resistant, now some of them are even brighter than regular bulbs (i.e. you get what you pay for, and a good reason to go with Chuck as you can trust the quality). But with the resistor you not getting the full power savings available with the LED. With a control unit, if available, you could have the total package.