Heater control valve location

This truck does not have one! Here's the description of your unit.
DESCRIPTION System is a blend-air type. Outside air is heated and then mixed with cooler outside air to attain desired temperature. System consists of a blower and an air inlet assembly, heater air distributor assembly and heater control panel. Blower, air inlet assembly and heater hoses are located on the engine side of dash panel while heater core and distributor duct are on passenger side. System has no heater control valve. Coolant circulation keeps heater core hot at all times. Operation of blower motor is controlled by the fan switch. The motor is connected in series with the fan switch and blower resistor assembly. On all Van and Suburban models, an auxiliary heater is available for heating rear of vehicle. The unit operates independently of standard heater and has its own set of controls.

This guide can help

https://www.2carpros.com/articles/car-heater-not-working

Heater hoses connect at the standard heater and extend from the standard heater to the auxiliary heater. A vacuum-operated heater control valve is installed in the inlet line in the engine compartment to cut off coolant flow when heat is not desired. See Fig. 7 and Fig. 8 .

Heater Core "” Plugged

WARNING: The heater core inlet hose will become too hot to handle if the system is working correctly.

Check to see that the engine coolant is at the correct level.
Start the engine and turn on the heater.
When the engine coolant reaches operating temperature, feel the heater core inlet and outlet hose to see if they are hot.
If the inlet hose is not hot:

the thermostat is not working correctly.
If the outlet hose is not hot:

the heater core may have an air pocket.
The heater core may be restricted or plugged.
If both are hot, the temperature blend door in the heater plenum may be broken or stuck.
You might also try a core backflush. Buy a flush kit, install the Tee in the return line(to pump), and disconnect the suppy hose, now backflush until water runs clear, I usually get two Tees, and flush both ways, a heater core is not linear, water path is much like a honeycomb. Hard to flush, radiator repiar shops use a pressure gun to clear them, and this codts about 100.00, but can save the time involved in replacing harder installations. If your core is easily accessible, changing it is the BEST answer!