Is the blend door vacuum ram working? You can find it under the dash, below the glovebox. If the blend door is stuck, it might not allow the warm air into the duct.
I second the thermostat though. Check coolant level, and temperature gauge after 10-15 minutes of driving. A temporary temp gauge with more accurate read out would help, but the stock gauge should be fine. It should steady right at half way or a little above/below depending on air temps. If it isn't getting there, replace the thermostat. About 5 bucks and some hand tools is all that takes.
The Black Beast 5.0
Better to Have Horsepower and not Need it, than to Need it and not Have it!
2000 Ranger Sport - 5.0L V8 - 8.8 LS rear