SUCCESS!!! Turns out there is a silly little spring that can become disconnected from the motor that controls the display mirror inside the HUD housing. I had to drill a hole in a bracket in order to re-attach the spring and now it works perfectly. If anyone else needs to do such a repair, I stumbled across the info while searching the web for an answer. Complete step-by-step instructions can be found here: HUD - Heads Up Display for Pontiac Grand Prix 2004 Repair Instructions