When you reinstalled the limiter switch brush set, you may have caught one of the brushes on the armature and broken it off. In this instance, the motor won't start on its own, but will run once given a nudge. The downward movement gets its nudge from gravity. You may need to remove the motor and then it's side cover to peek inside to see if both brushes are still in place. If they are, then while the motor's out, look up under the knob to see if you can see the rubber seal. If you can, you'll need to disassemble the case halves and install it into the top of the case half correctly.
Bill... I had the same exact problem as you for the last two weeks. I sent the site info on how to trouble shoot the problem. I followed the instructions and needed a new relay. I just finished putting in the new relay from the Fiero Store. Now the light functions normally. Problem solved.
[This message has been edited by jhgraham (edited 08-28-2013).]
In the event it is the relay, you can swap relays from one side to the other and if the problem goes with the relay, then order another. You can also get the actuator motor relay at your local parts store.
One word of caution, because the down stop of the relay is bad, the motor stoped by brut force. After putting in the new relay, turn on the headlights; you may need to manually turn the motor now about a half turn to initialize the relay, depending where the headlight stopped it may be more. Turning the knob causes the motor to turn on and go to the top position, open. Then turn off the headlights so the motor returns the headlight down. This will cause the relay to stop the headlight at the proper down position and not by brut force. Now the commands from the switch will be executed correctly both up and down. Verify several times by opening and closing the headlights.
In a sense, the motors do stop by brute force, even when they're working as designed. As the mechanism reaches the end of travel, whether up or down, parts of it come into contact with the stationary base. The motor continues to torque up and brute force moves the motor shaft either up or down until a set of breaker points inside the motor are opened and the opposing direction points are closed.