Where is the thermostat in a 1998 GMC Jimmy?

Cynthia, not exactly sure which engine your Jimmy has, here are the instructions for a 4.3 6cyl. The thermostat is located on the intake manifold at the end of the upper radiator hose. There is a little housing there with two bolts ,and thermostat is in it. "To replace it you'll need the following: 1) Thermostat 2) coolant 3) A closed container for holding drained coolant 4) possibly RTV sealant, keep it on hand just in case 4) 3/8" drive 10mm and 12mm deep socket,and a 3/8" ratchet 5) extension bar 6) 10mm open ended wrench 7) 1/2" deep socket. Take the air intake cover off. - Remove the two brackets which hold the throttle wires. Remove the two bolts securing the thermostat housing. Lift the housing and move out of the way. Remove the thermostat. If it remains in place, remove the rubber gasket from the engine. Be careful not to mar the metal, or get any gasket material in the opening to the cooling system. Stuff a rag into the opening if need be. Replace the thermostat. Torque the bolts down with 18 ft. lbs. of pressure". Refill cooling system and run the engine for a while to be sure it all worked out okay. Hope this helps, good luck.