Cadillac 2004 XLR Owner's Manual page 232

To open the fuel door, press the fuel door release
button. The vehicle must be in PARK (P) or
NEUTRAL (N) and the valet lockout button must
be in OFF.
5-8
The fuel door release
button is located on the left
side of the instrument
panel.
An alternate fuel door release is located inside the trunk
behind a panel on the driver's side. Pull the handle to
release the fuel door.