1999 Buick Riviera Owner's Manual - Page 60
1999 Buick Riviera Manual
Page 60 highlights
yellowblue CAUTION: Never do this. Here a child is sitting in a seat that has a lap-shoulder belt, but the shoulder part is behind the child. If the child wears the belt in this way, in 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. 1-54