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.
Step 2: Inside the Flasher
For this particular flasher, the 100k Ohm resistor connected to pins 4 and 5 will be removed and replaced with a 500k Ohm variable resistor.
Step 3: Remove the resistor
Step 4: Variable resistor.
Once you set your flash rate, you can measure the resistance on the variable resistor and replace it with a fixed resistor if you want. I left it variable because I plan to continue to add LED bulbs to my vehicle.
Step 5: Connections.
B = Battery +
E = Ground
L = Lights (This is a positive output. Connect the other lamp wire to ground.)
*These connections may vary for different vehicles!*