<-- Add side "glass" to improve the appearance and/or aerodynamics. The rule is there to tell you to use the windows (windshield, back glass, etc.) that came with the kit. Same thing for taillights, bumpers, etc. People had left those things off in the past to try and get better aero.
I used plastic from packaging (like on an RC car or a Barbie) to make the side windows on my car.
Use black spray paint on the insides of the glass to tint the windows -- that way the glass is dark, but still nice and shiny. A light coat of spray paint will let some light pass through, making it look a bit more authentic.