Have you tried turning the headlight **** clockwise or counter clockwise to adjust the brightness? I'm also curious as to whether you have a true 40th Ann or just a regular 04. ALL 2004 mustangs had the 40th Ann fender badges. But if its a true 40th Ann edition package it will have a 40th Ann badge by the lighter in the dash as well as written on the tach I believe.
I thought mine was a true 40th Ann when I got it due to the fender badges and was disappointed to find otherwise.
04 3.9l Convertible- Daily driver
94 5.0l Convertible- Work in progress
92 5.0l Hatchback- Another work in progress