Not good...as DODGE found out with their NASCAR 1969 Chargers, hence the
Dodge 500 with its "filled-in" rear window:
...also see
SAE paper #700036: "The Aerodynamic Development of the Chager Daytona for Stock Car Competition," by R. P. Marcell and G. F. Romberg, both of Chrysler Corp.
...lots of good info about front and rear spoilers, as well as backlight angles.