Window Guide Bushing by Just Miata.
The Mazda Miata NA and the Miata NB have a combination of plastic and rubber rear window bushing, depending on the year, that stabilizes the window as is it moves up and down while you crank or press your up and down window switch. Think of it like an outrigger on a canoe or training wheels on a bike. It is really important for window movement stabilization.
This bushing disintegrates over time in the early NA and NB years and falls into the bottom of your door or becomes sticky and resistant to movement in the later NB years.
Without the Window Guide Bushing, the window glass will shift and bind at the front edge of the glass inside the weather strip and slow window operation down as the window travels up and down. This sloppy slow condition puts stress on the regulator, can heat your window switch, raise the amps to the window motor, age the window motor prematurely, cause the regulator to fail, etc.
You can't buy this bushing from the dealership separately, you have to buy the entire window. Ouch!
Always lubricate regulator and regulator guides, rear window guides, and weather seal guides during installation, such as Honda Shin-Etsu weather seal grease to lubricate you weather seals.
Window alignment can also affect window actuation. The window can be adjusted at the window regulator fastening screws or the regulator mounting fasteners. The rear window guide can also be adjusted for and aft. The window should ultimately fit squarely in the weather seal track for optimal efficiency . The upward limit of the window. Observe the drip rail of the convertible top and the window fitment against the three weather seals.