GMC 2010 Sierra - Manual (Page 73)

If the belt stops before it reaches the buckle, when
using the lap-shoulder belt in a rear center seating
position of a crew-cab, tilt the latch plate and keep
pulling the safety belt until it can be buckled. 3. Push the latch plate into the buckle until it clicks.
Pull up on the latch plate to make sure it is secure.
If the belt is not long enough, see Safety Belt
Extender on page 2‑38.
If the latch plate will not go fully into the buckle,
check if the correct buckle is being used.
Position the release button on the buckle so that
the safety belt could be quickly unbuckled if
necessary.
2-31