All you need is 2 220 ohm resistors (although you could probably go 150 ohm and still be safe)
Take the back end apart and pull out the stock brake light. Use a dremel cutting disc to split the red reflector from the unit. Cut the wires going to the top two LED's (which are connected by a small circuit board). Cut the trace going between the 2 LED's. Solder wires to the + and - on the board for each LED. Connect the wires from the turn signals to these LED's using the resistor on the + line to lower the voltage. **EDIT** Reseal the light with clear silicone. **EDIT**
It blinks fast but I personally like it, more visible. Plus emergency vehicles around here use fast blinkers too
I thought about using a trailer kit do make each entire side be the blinker and have it operate as a 2 light system but decided that rewiring all LED's and cutting all the traces would be too much work. I also didn't feel like paying $85 for the clear kit with blinkers integrated.