There is no valve on these trucks - coolant flows through the heater core at
all times.  Check for a plugged heater core.  Disconnect the heater hoses
from the engine side of the firewall, and flush the core with a garden hose.
(Use some spare lengths of heater hose, running one into a bucket.  Flush it
in both directions).

Next, you should check the output from the water pump.  Stick both heater
hoses into a bucket, and have someone start the truck.  You should get good
flow out of one of the hoses.