The thermostat on a 1993 Ford Ranger 4-cylinder (2.3L) is located
at the bottom of the engine, on the engine block, near the water pump. It's usually accessed from the front or slightly lower front of the engine. You'll likely need to remove some parts to reach it, such as the fan shroud and possibly some hoses.
It's not easily visible without some disassembly. A repair manual specific to your year and engine will give you the precise location and procedure for replacement. A Haynes or Chilton manual for that vehicle would be a good investment.
