The VENT fan blower's multi speeds are determined by the voltage drop across some resistors. This resistors (they look like a coil of bare wire) are placed in the vent air stream for cooling.
This is from memory, I hope it is close:
In the front trunk, located just to the driver side of the AC dryer (that big silver canister), a connector plugs into the plastic duct work cover. On the inside of this duct work cover at the connection point are the wire coils used for dropping the voltage. There are 3 coils in here - 1 for each speed lower than high. If all 3 lower speeds do not work, something common must be wrong like the the ground.
I can't remember if the coils are replaceable from outside the duct work. If their base in riveted, removeal of the duct cover (and the AC dryer) may be required. Their base is made of a fiberous plastic/epoxy composite similiar to PC board material. (high temp resistance).
From: David Cole
As usual, David's comments are right on. I'm not sure about fieros, but here's some additional info (relevant to most GM cars) that might help.
One connection to the resistor pack is used for all speeds below HI. The low position will also utilize the resistor for the mid speed (in series). The high speed will use a relay to totally bypass the resistor pack. The high speed also uses a different fuse in many cases, but the relay is controlled from the same fuse that runs the lower speeds (since it is controlled by the dash switch), so no need to worry about the fuse if high works.
With the switch on, you can check for voltage at all terminals of the resistor pack (without removing it to look for telltale signs of burnout).
If you've got 12 volts at one or more connections and 0 at another, change the resistor pack. Otherwise, you'll have to track down the broken/bad connection.
From: Jeff Ely