With regard to the flickering, I believe they "dim" the incandescents not by using a lower voltage, but by "pulsing" the 12V to the bulbs. Incandescents heat up and cool off slowly, so a pulse train with a duty cycle of say 50% would reduce the effective power to the bulb, making it dimmer, but the pulsing nature of the current would not show up on an incandescent bulb.
Dimming by chopping the current like this is way cheaper than creating a whole new voltage with current capacity high enough for bulbs.
LED's, of course, are fairly fast diodes and will turn on and off as fast as you ask them to. A pulsed voltage on an LED will look like exactly that (I haven't actually seen the flickering but I'm assuming it's 10/second or so?). What would probably work to eliminate the flickering would be a moderately high-value capacitor mounted near the bulb's harness between the pulsed voltage and ground. The cap will charge during the "on" cycle and will hold the input to the bulb at near +12 during an "off" cycle. The cap wouldn't need to be anything special - a 25V rating to account for any inductance in the line and 10 or 15,000 mFd of capacity. Such a cap would cost maybe $4 or so.
I'd be happy to test this if anyone's interested. I just need a car (mine's a 2013) and maybe 30 minutes. If you're near San Jose CA and would like to try it, send me a PM.
So I contacted rDM on eBay as well. He said that currently the conversion cable is not available for sale in Korea do you know where else I might be able to find this cable?I can't edit my original post, but I just wanted to strongly endorse rpmtrade as the vendor. I've had some irregularities with other vendors.