For anyone who has added LED bulbs to their vehicles turn signals or brake lights.
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.
Here is the flasher I modified. Since I am using the flasher in my car, I just pulled this one out of another '97 Cougar at the junkyard.
There are two tabs holding the circuit board in, just pry them loose with a small screwdriver.