What about the airflow? Is it in Defrost vent regardless of setting? If so,
then check the vacuum hose to the yellow checkvalves by the brake master
cylinder. Make sure vacuum is good.

Do you have any other vacuum related problem?

I have an 82 300 TDT with version III of the auto climate control.  I
am guessing your '94 has a similar system.  On vehicles with manual
heat you move a lever that is connected via a cable to a valve that
controls the flow of coolant theough the heater core.

On an automatic system an electrically controlled valve opens and
closes when current is passed through an electrical coil/solenoid.

The valve can get contamination or sludge of some kind in it and then
stick.  Also there is a check valve that can get stuck open.

You should be flushing and renewing your antifreeze coolant on a annual
basis, or crap will build up in the system and start ruining things
like the auto control valve on your heater core.

Replacing the heater control valve is easy on my 300 TDT, it is in the
open on the firewall above the engine.   I just replaced mine and the
part was less than $200.

Paul Fretheim