Buick 2004 Rendezvous Owner's Manual page 67

Second Row Center Position Bench Seat with Third
Row Seat Folded Down
If the vehicle has a third row seat and the seatback is
folded, there is an anchor on the back of the third row
seat for the second row center position bench seat.
Third Row Passenger's Side Outside Position
There is also a top strap anchor for third row passenger's
side outside position. Locate the anchor symbol on the
flap of carpet behind the seat. Lift up the carpet to access
the anchor.
1-61