I just bought a 2005 MGM LS. I have two others, a 2000 and an 2004. For some reason, the trunk lid on the 2005 does not raise as high as the trunk lids on the 2000 and 2004 cars.

 

The trunk lid opening height is adjustable -- by moving the transverse rods that run between the trunk hinges.
Open your trunk and study the way the rods work while you move the trunk lid up and down.
You should be able to determine which rod (s) to adjust, and how many notches of adjustment to make.
It's strictly a trial-and-error operation, but you should be able to fix it easily.
Good Luck !!