If you turn on your 1999 mustang v6 car and the right signal light stays on and doesn't blink and you switch the car off and then the left signal light stays on without blinking, then you need to see a mechanic.
Defective turn signal switch.
If it's blinking, it's a turn signal.
A signal that is blinking fast usually indicates a bad bulb or bad flasher. Check to insure all of the bulbs burn, then check the flasher.
One of the bulbs is out. Replace it.
change the flasher to a slower blinking brand. They are cheap and you can try a few brands
It has been my experience that a rapid turn signal indicates there is a burnt-out signal light bulb (either front or rear) on the side of the rapid blinking signal. If the right signal is blinking rapidly, the problem is in the signal lights for the right side (front or rear) of the vehicle. Turn the rapid signal on and step out of the vehicle to find which bulb is not lighting.
The monitor light keeps on blinking for some time after switching off to indicate immediate loss of input signal. When the computer is switched on, the LED stops blinking and changes to green, indicating good signal reception.
The LED light is blinking fast because it is receiving a rapid electrical signal, causing it to flash quickly.
is the car on? do you have the signal on? is there a bulb? are there wires connected to the bulb? or just go to a mech and have him figure it out. simple.
it needs to be replaced
Defective flasher relay.
Check for a burned out bulb or could be the turn signal flasher going bad