There's a turn signal or running light out on that side, either front or rear. Turn on signal, get out and look. If both turn signals are working, chances are its the running light.
you need to change the flasher can for the signal lights
When both of the turn signals (indicators) of a car blink simultaneously, it is usually because the hazard lights are turned on.
I replace my flasher fuse.Now they light up but won't blink.
If both your left and right trailer signals blink at the same time there is something wrong with your wiring harness.
The flasher relay is defective. Replace it.
I would check the parking lights,turn signals and side lights front and back.Its got to be one of them. It either don't blink,or don't come on at all or might have a short in the bulb itself. I own a Pontiac sunbird 1994 and if the problem is inside the cab more than likely I would say that its got to do with the signal selector and the relay switching in the steering colum its a hard part to come by. Literally you have to rip arpat the steering colum to get at it.
It vibrates and sends signals to your brain quicker than a blink.
one of the lights is not working
If you have a bulb that is burned out then it will blink quicker than normal. If all the bulbs are fine replace the turn signal flasher....usually located in the fuse box. Some cars have it tucked up next to the steering column under the dash. I'm not sure about your specific car as to the location.
The turn signal relay may be the problem if the turn signals in a Ford vehicle are blinking slowly. This relay is usually part of the electronic control module located near the engine.
You can buy special bulbes that blink your lights slowly or you can computerize them.
Yuo may have a faulty relay, or one or more bulbs are blown.