If it's moving past the 1/4 mark, you should get some sort of heat. Not super hot, but pretty warm at the least. I would suggest flushing out the heater core. They like to clog up with gunk and stuff over the years. My process is to simply disconnect the heater core hoses where they connect to the water pump and/or engine and hook a water hose up. Point the hose that doesn't have anything hooked up away from all electrical stuff (preferably outside of the engine bay). Turn the hose on, starting with very low pressure first, and let it start pushing the stuff out. Usually, you'll see a bunch of muddy crap come out. Once the mud gets out and the water starts to clear up, turn the pressure up so it's really flowing good and let it run for a couple of minutes. Then reconnect the hoses, top off the cooling system, and try the heat again.
If that's not it, then you'll want to check for issues with the blend door and it's actuator.
You might also try simply disconnecting the battery for at least 15 minutes. Then reconnect it. Turn the key on, and wait a minute. Then start the truck and try the heat after it's warmed up. See what happens.
-- Explanation: The actuator in this truck is electronic (I think, at least that's what AllData says), meaning it's controlled by a computer. Sometimes, as parts like the actuator and door wear, their positions change a little bit. You might just need the computer to go through the relearn process in order to compensate for wear and tear. Disconnecting the battery for 15 minutes should force the computer to relearn when you turn the key on.
Try those things and let us know how that goes. And don't worry about how long it takes you to respond, I'm on here daily doing my thing. So it makes no difference to me.
Thursday, January 25th, 2018 AT 8:38 PM