Hummer 2003 H2 Owner's Manual page 144

Crossrail Channels
The crossrails have built in channels to allow you to
attach other items designed for this system such as
basket luggage carriers, bike racks, ski racks, etc.
To use the crossrail channels do the following:
1. Use the included key to unlock the crossrail end cap
by turning it counterclockwise. If you ever loose a
key, your dealer will be able to help you obtain a
replacement.
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3. Peel back the rub strip from the crossrail.
4. Slide the accessory you are using into the crossrail
channel and secure it as the accessory instructions
direct.
5. Place the crossrail end cap back on and lock it
with the key.
You will not be using the rub strip when you use
crossrail accessories. When you remove any crossrail
accessory, be sure to reinstall the rub strip. To do this,
press the rub strip in place until it seats back into the
channel of the crossrail.
2. Pull the end cap straight
out from the crossrail.