About 4 years ago I had my GT inspected, and since the passenger side headlight didn't always go all the way up, my (former) mechanic remedied this by somehow loosening the headlight door assembly or whatever you'd call it. The car ended up sitting for the next 4 years, but I've had it back on the road for a few weeks now, and the problem is obviously that since the door is no longer as tense as it should be, it is popping up at Highway Speeds (anywhere from 50 mph and above).

I really don't know what he did to it as it was so long ago, and I compared it to the driver's side door to see if there was anything obvious, but I can't figure it out. How do these headlight door assemblies work? What should I be looking for?

They use torsion springs, maybe he removed one of them. they are rods. You could replace that part of the headlight door and just swap your painted cover.

Those rods are still in tact, that's what I thought gave them tension when comparing the passenger's side to the driver's side, but it doesn't seem as snug on the passenger side, you can wiggle it around a bit. I don't know what he did!

maybe he bent one of the rods so it doesn't have as much tension ??? How about replacing the rods with a used set?