Quarter window on automobiles and closed carriages may be a side window in the front door or located on each side of the car just forward of the rear window of the vehicle. Only some cars have them. In some cases the fixed quarter glass may set in the corner or "C-pillar" of the vehicle.
Older vehicles frequently separated their side windows into two separate pieces. The main window resembles the ones we know today, and could be raised or lowered through a handle on the door. The second piece is triangular in shape, opened outward, and listed in manuals as a "quarter window" or "quarter glass".
For the '87 model year, Ford switched to a much cleaner-looking rear mustang quarter windows assembly for Mustang hatchbacks and hardtops. Molded from vulcanized rubber, with the actual window glass embedded, the one-piece assembly was quite complex for a mustang quarter windows, but it served its purpose by effectively blending the roof into the rear quarter-panel. But over time, with many hours of hot sun baking the side windows, the exterior rubber deteriorated, leaving a pitted and discolored window perimeter.
It is unfortunate that the "quarter windows" on Mustangs have begun to deteriorate, and are rather expensive to replace properly. Some owners have moved to drastic measures such as paint and patch kits, but these only work for brief periods of time.
Recent model restoration resources have filled a niche with the ideal answer: totally new 1987-93 rear /"quarter windows for mustangs"/ made by an original-parts maker. Costing $599.99 a pair and offered for hatchback as well as hardtop models, these windows are the same as original assembly-line Ford products, down to details like the Mustang logo on the window.
Replacing the rear mustang quarter windows is a basic R&R procedure. Getting to the nine retaining nuts for each ford mustang windows are the most difficult part; basically, for a hatchback, you have to remove all of the plastic trim in the rear of the interior. But, for the most part, you're dealing with small Phillips head screws, although you'll need the correct socket for unbolting the front seatbelt mechanism on each side of the car.
Quarter windows, i.e. the windows found just forward of the rear window of the car, present an obstacle for the Ford Mustang enthusiast. Starting in '87, Ford went with a new one-piece vulcanized rubber quarter window. If you need to replace your Ford Mustang windows, specifically the quarter windows for mustangs year 87-93, you can get a pair for $599.99 which are easy to install. The Mustang quarter windows are made by an original parts manufacturer, and are an excellent and affordable option to "patch-and-paint" kits. These measures simply do not work long term.