Not sure if that model uses a flow switch in the heater coolant line to control flow or not. Check the hoses running from the heater core and see if there isn't a valve in one of those lines with a vacuum line or cable connected to it. If not then you don't have a control valve and heat to the inside of the truck is controlled by a diverter door in the plenum. If you don't have a valve both lines should be hot at the heater core. If not then you probably have a plugged core or coolant isn't flowing through the hoses. Make sure the coolant level is topped off in the radiator. If you do have a control valve make sure the vacuum line or cable is connected. If it is and you don't get any heat, disconnect the vacuum line to the valve and with the heater controls to heat and highest temperature check for vacuum at the vacuum control hose. If it is a cable check to see if the cable is moving the control valve. All of these tests need to be done with the engine at idle. If no vacuum then you need to track down a vacuum problem. If there is vacuum reconnect the hose and see if both hoses are hot indicating coolant flow through the heater core. If no flow probably a bad control valve or plugged heater core. If the heater core appears to be plugged you might be able to open it up by removing both hoses from the core, getting a short piece of heater hose and attaching a plastic female garden hose connection and connecting the hose to it and turning the water on very low to see if that clears the blockage. If it is blocked tight you will need to replace the core. If water flows repeat the process through both sides of the core until water flows freely. Don't turn the water on full force or you will probably pop a leak in the core since the seams are soft solder and can't take much pressure.

John