GMC 1996 Sierra Owner's Manual page 59

Never do this.
Here a child is sitting in
a
seat that has
a
lap-shoulder belt, but the shoulder part is behin
the child. If the child wears the belt in this way, in
I
a crash the child might slide under the belt. The
belt's force would then be applied right on the
child's abdomen. That could cause serious
or
fatal injuries.
Wherever the child sits, the lap portion of the belt
should be worn low and snug on the hips, just touching
the child's thighs. This applies belt force to the child's
pelvic bones in a crash.
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