The fuse box is under you back seat! The rear signal flashers have their own control board located in the taillight housing. I have had to replace mine before. I am not aware of any fusebox under the back seat. I will have to check this out.
Check for a burned out bulb or could be the turn signal flasher going bad
we are going to need to know the year, make and model to answer that one. It varies from one vehicle to the next. Please ask a new question with more information.
Sounds like flasher is defective and will soon need to be replaced
You are not going to like this answer! That flasher is located on a block with the fuses high up in the dash behind the glove box. I'm not sure if you can get at it or even see it without removing the glove box and related trim. Block is pressure fit to a metal stem and can be removed and pulled down to install new flasher relay.
I would start with replacing the signal flasher relay.
Most likely a bad turn signal flasher
The flasher is underneath the steering wheel. Looking from underneath the steering wheel, it is located to the left near the stereo. It is hard to describe the exact location, just pop your head underneath the steering wheel, and, looking up, there is a little compartment and the flasher is attached all the way to the left of that compartment. The stock flasher on my mustang was yellow, maybe yours is yellow too. It is a plastic rectangle with a protrusion on one side for a clip. The flasher slides onto a flat metal rectangle, you're going to need to yank it off there to replace it.
It could be thast your turn signal flasher module is going out
You probably have a burned out bulb. The turn signals both use the same flasher.
Wrong bulbs installed? Defective or corroded sockets? Bad turn signal flasher?
The first thing I would do is replace the flasher. The flasher is what controls the on / off of your blinkers, giving you the blink blink. If that doesn't work, it sounds like your turn signal switch is going bad. GOOD LUCK
The turn signal flasher is going bad. Pretty common. The part is available at most auto parts stores, finding the flasher module, and replacing it can be difficult. The manufactures like to hide them.