OK - I'll try not to get to in depth and keep it simple -
IF you are wanting an easy install, firing the subs backwards in the G body is the best way to go.
IF you want to do a more involved install, firing them forward is the best way to go BUT - it is imperative that NO sub bass gets into the trunk. ie: the sub has to be perfectly sealed to the cab of the vehicle. This is quite involved and takes alot of care, materials and attention to detail but WILL result in the most accurate bass reproduction overall.
The reason is what is commonly referred to as standing waves.
When a sub is fired forward (not sealed to the cab), the sound wave travels in ALL directions (forward, backward, up, down etc...) The result is alot of waves bouncing around that will interact in a negative manner (in and out of phase). The biggest culprit of standing waves is the wave that bounces off the rear of the car and then comes forward. When this happens, overall bass reproduction is greatly reduced (out of phase to itself).
When the sub wave is sealed into the cab only (fired forward), standing waves are greatly reduced resulting in ultra clean/tight (in phase) bass reproduction because the sound wave can't go backwards there fore you greatly reduce any standing waves.
When a sub is fired backward the same thing happens, the wave is essentially "bounced" off of the rear and then it comes forward. Cabin gain is increased as well resulting in some increase in sub bass volume.
The result is excellent bass response with some loss to overall accuracy. A rear fired sub will also have a tendency to sound some what "tubby" and sloppy BUT, it seems that this is what most people prefer.
Under NO circumstances would i suggest the sub fire up at the trunk lid.