Also, has the coolant been changed according to service manual?
You have three possible issues if the blower works fine.
1. Air in system. Solution- vacuum fill system
2. Plugged heater core. Test- check the in and out hoses on firewall for temp difference while truck is warming up. Temp differential will say heater core plugged. Solution- replace heater core is recommended, other solution is to find someone with a flush gun.
3. The actuator door is stuck/ motor non functional. Test- listen for motor movement or find wire and verify power. If verifying power, you must verify motor operation. If motor is good, door is jammed.
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