90 percent of the problems with Ford Rangers and wipers, headlights, fog lights etc. Can be traced back to the dimmer switch on the steering columb. For some reason ford went cheap on this component and the switch tends to melt down under normal electrical lode over time. I would be willing to bet thats the problem. Check the fuse first, then the relay, if those both check out the chances are its the switch. You could try replacing the wiper motor if that does not fix it then its definately the switch. New switch will run you 80 bucks. don't go to pick and pull for this part as this eventually happens to all ford ranger dimer switches. Shell out the money for the aftermarket replacement and you will not have any more problems.
First you need to check the fuse, then check the switch and the last thing I would check would be the wiper motor. If it sounds like the motor is running but the wipers are not working then you have rubber grommets located where the wiper arms connect to wiper motor that are probably bad.
Mine don't work if you find out post it big and easy 2 find!!!