Fast blinking signal light, but light bulbs are working

There should two separate bulbs for marker and blinker. It sounds like a bulb is burnt out, bad ground, melted socket or a wiring anomaly. Are you certain it's supposed to have 57 series dual element bulbs for blinker and markers?

Get the left signal to flash fast then walk around the car to see which light isn't flashing then change the bulb or socket if they all appear to be flashing (front and rear) turn on the right turn signal then look into the front and rear lens and count the number of bulbs flashing, now do the same thing on the left. Many times there's more than 1 bulb in each lamp. I hope this helps. Take care.

Check one of your signal bulb is out check all 4 of them

Generally speaking, symptoms of a bad or failing turn signal bulb are usually the culprit...in your case, either it's the front left or rear bulb. Varies for different vehicle makes...common signs include:

1. turn signal light staying on solid
2. or blinking very fast
3. or either sides of the bulbs stop blinking altogether.

To test this, make sure you reconnect everything you disconnected.
Turn the left signal light on; go to the front of the vehicle, is the turn signal bulb illuminated? If it lights up, even though it is not blinking obviously the bulb is okay. If it doesn't, replace it- problem solved. You'll do the same procedure for the rear turn signal Is that lit up? If not replace the bulb and the problem is solved.
Another common failure is when the turn signal blinks really fast, same procedure as mentioned above.
If both bulbs for the left side and right side illuminate but don't blink, the problem is in the steering column turn signal switch.
If the left or right side blinkers don't work properly at all...then check the integrated hazard warning/turn signal flasher relay- that may be defective; again, that's usually the case if both sides of the signal light don't work.
If you are blowing any fuses related to the flasher system, you have an electrical overload in the turn signal circuitry.

Sounds like an indication of a burnt out bulb. Check the front bulbs

Maybe nothing, but check all your turn signal lights, front and rear. Often, a fast blinking flasher indicates a burnt bulb or one not working somewhere. If all are working, it just may be the nature of the turn switch and the flasher relay for it.

sounds like burnt out bulb. This is usually an indication when the bulb blows. Turn on blinker and check front and back to see which bulb is out..

sounds like you got a bad bulb. turn the blinker on an step out to look at them. there should be a blown one.

the fuse is broke

I would start by checking the blinker light bulb on the left side front and or rear,some vehicles will blink real fast and some wont blink at all when a bulb goes bad.