PhilD is right, you need patience. It was damn near impossible for me to do it without removing the A-Pillar cover. Once that cover is removed you can see everything and the whole process will go easier.
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'03 Hummer H2 Adv./Lux. Pkgs. Sunset Orange W/Sunroof
Air Spring Susp. Pkg. + 3rd. row seat & DVD System.
CompuStar Auto Security/Remote Starter.
http://www.haugens.com