Will a headlight from 1999 Pontiac Grand Prix fit 2003 Prix?

While both are Pontiac Grand Prixs, there's no guarantee a 1999 headlight will fit a 2003 model. The body styles changed significantly between those years. It's highly unlikely they'd be interchangeable. You'll need to verify compatibility using the part numbers or consulting a parts store's online catalog (like AutoZone, Advance Auto Parts, etc.) using the VIN of your 2003 Grand Prix.