First step: check that your radiator is full (open cap with engine cold) and you have fluid in the overfill tank as well.
If that checks ok, then with truck running and heat is working, feel both heater hoses -- they will both be hot to the touch. When the system turns cold, keep everything running open hood and feel same two hoses.
If they are no longer hot, or one is substantially cooler than the other (the one on the side that runs to the valve), then you have a mechanical issue with either the valve or an obstruction in the heater core itself. Most probably the heater valve (located in the heater hoses on the front of the block under the hood), but sometimes you can have a partial blockage in the heater core that gives the same symptoms. The valve is controlled by a vacuum line that runs from it to a vacuum ball located in the right fender well. Check for tight fit and any cracks or breaks in the line or the ball itself. Also look for any leaks at the valve indicating a mechanical failure. Has anyone put leakstop in the system? If so you may have a build up and restriction or clog in the valve as well.
If both hoses are still hot but you get no heat while the blower is pushing air out the vents, then you have a different issue -- blend door related.
