There aren't too many things that are more fun for a car lover than putting down the top of their convertible. Unfortunately, your rear plastic window doesn't feel the same way. Constant exposure to the sun can make the window become cloudy and opaque over time. The constant motion of the canvas top moving up and down makes the plastic brittle. When this happens, you have to replace the window. This is an easy task if you have a canvas top with a zipper for quick removal. If not, you'll need to grab a thread and needle.
Remove the canvass top from the car. To do so, unscrew the frame from the top of the car using the screwdriver and possibly a wrench. Designate a safe spot for all the loose nuts and bolts so you don't lose any.
Find the threading on the canvas top near the rear window. This is usually somewhere beneath the plastic window.
Cut along the threading to create a hole in the canvas top. The threading is actually holding two pieces of canvas together with the plastic window between the two pieces.
Remove the old plastic window and insert the new one, making sure that it is centred.
Use the thread and needle to sew up the threading you had cut into. Make sure to sew it strong enough to hold the plastic window in place.
Attach the canvas top back to the convertible.
Glass rear windows are considered a more durable alternative to plastic, though they can cause some sagging in the canvas top due to the glass' weight. Keep your top upright when you store it to prevent bending, which can create stress fractures and lines in the window.