Since the LED bulbs use less Amps than normal bulbs, the flasher unit thinks there is a bulb burnt out and doubles the flash rate. This instructable will show you how to alter the flasher module in your car for variable flash speeds.
Note: This is for electronic relay type flashers only. This will not work on thermal flashers.
Step 1: The flasher.
There are two tabs holding the circuit board in, just pry them loose with a small screwdriver.