1998 ford taurus left turn signal blinking fast

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.

bad lamp is my guess,

indicates a problem in the harness to the rear harness
cut wires , incorrect additions to harness
possibly along the chassis or under body

remove rear left lens and install new turn signal bulb.

there is a bad ground on the headlight

Sounds like a bad ground to me, although i am no expert.
i would be checkinh all connections and take it from there, a little wd40 might just solve it short term.

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.

any increase in bink speed is direcly related to a bulb being burt out.

replace rear left globe and this will solve both problems cheers MAL

sounds like a bad ground through one of your lights, the right front or rear turn signal bulb.