1988 ford mustang. Turn signals do not work

You did not say if its one turn signal or all turn signals that dont work. It could be a bulb.One bad bulb will cause the others not to work , normally one side of the car.Mostly left and right sides of the car is on a different circuits.
If it is all of them not working , it could be the switch in ur steering colum thats bad.Its the switch that the handle u move to turn the turn signals on and off.You would normallytake off the steering wheel to fix that switch.But its a bigger jpb , a pain.So make sure its not a bulb or a fuse first and make sure ur useing the right amp fuse.If its not that.Then id check the conection where the bulb screws into the socket.There can be corrosion there and that can cause a bad conection.Besides a bad bulb thats always been the number one reason my turn signals have not worked.You just have to keep eliminating the possibilities. Keep checking.