Cadillac 2007 STS Owner's Manual page 240

To access the passenger compartment air filter,
do the following:
1. Pull back the rubber hood seal from the edge
of the leaf screen vent cover.
2. Remove the three fasteners that hold the filter
access cover in place and slide the cover off.
3. To access the filter, remove the black plastic
water deflector by lifting the outboard edge of
the deflector to release the retention tab.
4. Lift the inboard edge of the deflector to
release the retention tab.
5. Lift and slide the water deflector toward the
inboard side and remove it.
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6. The top edge of the filter should be visible.
Reach in and lift the filter out, pulling upward
and toward the front of the vehicle.
7. Insert the new air filter by sliding it back into
place. Make sure the arrow on the filter is
pointing toward the passenger compartment.
Reverse steps 1 through 3 to reinstall the water
deflector, filter cover and the hood seal.