Chevrolet 2000 Blazer Owner's Manual page 22

To return the head restraints to the upright position,
reach behind the seats and pull the head restraint up until
it locks into position.
1-10
On two-door models with an inside mounted spare tire,
the driver's side rear seat head restraint must be lifted
and held upright as the seatback is raised. Otherwise, the
head restraint will hit the spare tire and prevent the
seatback from properly latching.
Push and pull on the head restraints to check that they
have locked in the upright position. If they haven't, have
them fixed immediately.
For information on adjusting the rear seat head
restraints, see "Head Restraints" in the Index.