[lukecp]

Yes, like cdowns said, the valve controlling heat flow could not be working (assuming this is a non AC truck).

You can look at the flapper valve from inside the cab....it is where the heater control cable goes into the heater box, under the dash in the passenger side footwell. There is a little plastic piece that seals the rectangular hole where the control cable goes in, pop it out and have a look. Make sure the cable is still attached to the lever on the flapper valve, and that it is opening and closing all the way.

If that checks out, you proably have a heater core plugged with pine needles or something. You need to remove it....and proably replace it too. A new core is only around $25, and as long as it is out it would be a good idea to replace it.