GMC 2006 Yukon XL Owner's Manual page 68

Second Row Seat (Bucket Similar) — Extended
For Regular models with third row seating, there is
one top tether anchor located at the bottom rear of
the seat cushion that can be used for either the center
or passenger side seating position in the third row,
but not for both. Never install two top tethers using
the same top tether anchor.
1-62
Models
Third Row Seat — Regular Models
For Extended models with third row seating, the top
tether anchor is located at the bottom rear of the seat
cushion for the center seating position in the third row.