A/C problem

Sounds to me like the fan has gone bad on the lower speeds.

There's a pack of resistors used to slow the fan down for slower settings. Chances are one or more of these are bad.

The following link may be of some help:

This particular episode was not directly about your problem but if you scroll down to Fig. L - it will show you what the resistor looks like for a blower motor. What happens is: When you set the switch [on your dashboard] to the low or medium setting - the current to your blower motor passes through this resistor and the blower motor doesn't run at full speed. If you choose the high setting on your [dashboard] switch - the resistor doesn't "slow" the current/voltage [so to speak] to the blower motor and you experience your current situation. Typically the blower motor resistor is located in 1. the engine compartment [usually near the firewall] or 2. in the vehicle [usually under the dashboard].

With the Taurus I believe it's located under the dashboard on the passenger side.