[Mark42]

The only other exterior option was the short-lived GTA Notchback of 1988-89. Seeking something to further differentiate the GTA models from the rest of the Firebird line and in an attempt to lessen the "greenhouse effect" from the big rear hatchback glass, GM came up with the idea of an optional flat-glassed rear notchback hatch lid (RPO AA8), similar in appearance to the 1968-1976 Chevrolet Corvettes. Beginning in December of 1987, this was a GTA-specific option and could not be ordered on any other Firebird (or Chevrolet Camaro model).

Production records show a total of 625 units were built during 1988 and 1989. The option was actually canceled before the start of production for 1989; a dealer bulletin in August 1988 announced its demise, instructing dealers to recall any and all GTA orders with the Notchback code on them. Still, PMD records show that one single car was built for 1989........ no doubt the car pictured above from the 1989 Pontiac brochure.

Today these GTAs are very rare sights and are highly collectible, even though the style really did not catch on during their brief production run. These cars were the only GTAs not to use a variation of the rear aero-wing spoiler, since the notchback design changed the layout of the hatch lid. PMD engineers simply installed the standard flat Firebird spoiler instead of the usual aero-wing unit.

From here: http://www.gtasourcepage.com/modelfeatures.html