To be certain, you should:
1. Consult your owner's manual: This is the best source for the precise location.
2. Check your fuse box diagrams: The diagrams often show the location of relays as well as fuses. There might be multiple fuse boxes.
3. Look online: Search for "1999 Ford Ranger flasher relay location" along with your specific trim level (e.g., "1999 Ford Ranger XLT flasher relay location"). You might find pictures from others who have located it.
Remember to disconnect the negative battery terminal before working with electrical components.