GMC 1999 Sierra 1500 Pickup Owner's Manual page 24

Seats & Safety Belts
Rear Seatback Latch (Except Bench With Armrest)
K2106
The rear seatback can be folded forward to let you reach the area behind it.
Your seatback will move back and forth freely, unless you come to a sudden
stop. Then, it will lock into place.
There's one time the seatback may not fold without some help from you.
That's if your vehicle is parked going down a fairly steep hill. If this happens,
push the seatback toward the rear as you lift this latch. Then the seatback
will fold forward. The latch must be down for the seat to work properly.
Rear Seatback Latch (Bench Seat With Armrest)
The rear seatback folds forward to let you access the rear of the cab.
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