I don't know why, but I was able to leave all factory wires
intact and use no additional inline components by using a
flasher made for LEDs.

http://superbrightleds.com/flashers.htm

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FWIW, I had terrible luck with these LED bulbs

http://superbrightleds.com/specs/1156-xLX5.htm

They fell apart from vibration in short order
Perhaps they've been improved by now