heat only vents to windshield

When i put the blower motom switch in any position(floor,upper vent or windshield) it will only bolw on the windshield. Any suggestions, Thanks. Mike

My AC switched to the windshield with load on the engine a while ago and I eventually found out that a vacuum check valve had a hole in it. It was simple and inexpensive to replace. The vacuum lines run from the intake manifold and split off to a number of locations, one apparently the AC/heat switch. After auto parts shops couldn't help I found replacement vacuum check valves for both on my system from the US Plastics website for a few dollars each. The dealership wanted about a hundred to fix the problem...