Cadillac 2004 XLR - Manual (Page 34)

You can use these lower anchors to install the child
seat instead of using the vehicle’s safety belts if the child
seat has the necessary attachments (A, B).
However, your vehicle does not have a third anchor,
called a top strap, or tether, anchor (C). If the
instructions that come with the child seat say that it
must be secured at all three anchors, do not use that
child seat in this vehicle. See
Top Strap on page 1-27.
1-28