Cadillac 2008 DTS Owner's Manual page 189

The access panel for the passenger compartment air
filter is located under the hood near the windshield,
on the passenger's side of the vehicle.
To access the passenger compartment air filter, do the
following:
1. Use a tool to gently pry the cover up. If your vehicle
has tabs that allow you to unlatch the cover with
your fingers, you will not need a tool for this step.
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