Harm?  No, but you should drain the heater core. With no circulation
through the core, you are asking for trouble.  If the core is the
original('95), that may be a moot point anyway since 10yrs is the mean
lifetime of a heater core.  I would pull both hoses off the core and
couple them together with a piece of pipe.  I suspect your main
complaint is getting heated air through the ducts when it shouldn't be??
If your system uses a cable to operate the blend door, is it properly
adjusted?  If it is, then most likely the blend-door is the trouble.
Ford use to use, and may still, plastic rivets to attach the door hinges
to the plastic duct-work.  The plastic rivets eventually break/wear out
and the blend-door does not close properly.  When the door doesn't seal
the heater core off from the duct-work, heated air will escape into the
ducts.  Fixing that door ain't much fun, but not too difficult either.
I had to buy a new door and install it.  I used Aluminum rivets instead
of the plastic ones.

Probably more answer than you wanted.....

Dave S(Texas)

mail box full to defeat spam....which is what the mail box is full of!

...one thing I have found out, there are soooooooooo many experts on the
net.........

Joe