See if the circuit board says 6122 on it. If so:
Remove the connector from terminal strip J3 (which goes to the box's speed switch).
Figure out what the other switch connects to and remove that (you may have to remove the contacts from the Molex connector). (I've never installed one of the boxes, so I'm not up on how the other switch connects.)
Connect a wire (18 or 20 gauge will do) to terminal strip J3 pin 1 and another to J3 pin 2. Connect these wires to the half-moon such that when the switch is in the Fast position the switch is closed and connects the wires (and thus pins 1 and 2) together.