97 Taurus A/C hot -cold door

The electrically driven door ( under the dash, passenger side ), when the heat selection is made, runs continually opening and closing the door. Is this a electric or vaccuum line problem?

It’s an electrical issue or a mechanical issue with the door. Vacuum is not involved in the control of the blend-air door for temperature control. Vacuum is used for the doors that select where the air comes out and if fresh or recycled air is used.

When the cold air selection is made the electic motor stops activating the door.

That would tend to indicate that the door is unable to fully move to the heat position. You would probably do best to try and find a Taurus specific board to get help with the problem.