Pick-up trucks are unique to the auto glass industry in that they don't have the same sloping single-piece back windshields that are found on cars, SUVs, or minivans. Pickup truck manufacturers use either a single piece of vertical glass or three to four pieces of vertical glass in a frame. The latter is referred to as a slider. A slider gets its name from the way that the middle section of glass is able to slide open.
Rear truck window sliders come in a variety of shapes, sizes and options. Traditional sliders use a metal or plastic latch to unlock the slider from the inside. From here they can be pushed open by hand. Three panel sliders have a single center piece of glass that slides open. Four panel sliders have two sliding pieces that meet in the center of the frame. This allows for a larger opening.
Newer rear sliding windows are electronically operated with a switch and do not need to be unlatched first. This is a convenient option in extended cabs and crew cabs where occupants in the front seats cannot reach the rear window as easily. Some more unique options for pickup truck rear sliders come from Ford and Toyota. 2015 and newer Ford F-150 models offer a flush sliding rear window, giving this back glass a unique look. Toyota's Tundra Crewmax features a rear window that actually rolls down all the way into the truck's body for an open-air feel.
A sliding rear window may seem like an odd, or even unnecessary option at first, but longtime pickup truck owners know that it serves many not-so-obvious purposes. When hauling long objects like pvc pipe, wood trim, carpet, fishing poles and the like, the opening can act as an extension of the bed-- or vice-versa as an extension of the cab into the bed (always make sure any objects carried this way are safely secured and do not impair visibility and safety).
Another benefit of a truck's slider glass is extra ventilation. There are still many pickup trucks on the roads without AC, and even those with the option still enjoy fresh air once in a while. Opening the slider not only allows for extra air flow, many truck owners also report that it lessens wind noise. Lastly, those with camper tops will like that it allows them access to the back of the bed, which would not be possible otherwise.
Trucks are meant for work. And anyone who has used a pickup for what it's meant for, knows that things break. Because of its vertical positioning, the back glass slider of a pickup is vulnerable to breakage. This is usually caused by impact from tools, equipment, or materials that are being hauled. Always be cautious when loading objects into the bed of a truck.
The issue you're experiencing may not be quite so serious though. Other issues can occur with sliders as they age. The slider may get off track and become stuck. Other times gaskets around the slider can deteriorate, causing water and air leaks. For those with electronic sliders, the motor or switch may fail, leaving the slider stuck open or closed.