Where is the thermostat located on 1999 jimmy v6 4.3 engine?

The thermostat on a 1999 GMC Jimmy with the 4.3L V6 engine is located on the engine block, at the rear of the engine, on the passenger side. It's often tucked in a somewhat difficult-to-reach area. You'll likely need to remove some components (possibly hoses and possibly even the fan shroud) to get good access to it. A repair manual specific to your vehicle year and engine will show the exact location and removal procedure.