1999 Windstar No Heat Blend door?

I'm new to this forum, but I could really use some help here. I have a 1999 windstar and the heat does not work in the front of the vehicle. I have ruled out the thermostat and the heater core. I believe it is a blend door or some sort of electronic switch. Where do I begin to look or find detailed instructions? I have a Haynes manual for the vehicle, but it does not seem to have any useful info on this problem. The blower works fine and the switch that controls the defrost, feet, panel, etc also works fine. Only cold air will blow out of the vents.
I live in a rural area and two of the local mechanics told me that they did not have the diagnostic tools to check. One of the local mechanics said that all of the vacuum lines appear to be working normally. I would prefer to avoid the Ford dealer due to the distance and cost. I have not been able to find any details diagrams or potential ways to remedy my problem. Thanks.

By saying you've eliminated the heater core. Does this mean you've flushed it seperate from your coolant syatem? They do tend to clog.

Denis

Do a search on "blend door" in this forum ... it's been a hot topic every year as the weather begins to cool.

I appreciate the replies. I ruled out the heater core because both the inlet and outlet are hot when the vehicle is running. Also, both of the local mechanics that looked at it thought that it was an electrical issue as well.
Does anyone know where to find a diagram of the ductwork? I have read several items including a similar post, but I'm not even sure where to look for the duct that may have the blend door problem. One other thing I should note is that my van does not make a clicking sound when I move the switch as some of the other people mentioned with their vehicles. Would I still be wise to check the actuater first? If so, how can I rule that out or determine if it is not working vs. the blend door being stuck? Thanks.