VL Turbo is 100% correct.
There are a number of thing is understand when looking at doing this particular job.
1. rear seat must come out.
2. Factory holes are for 6 inch speaker only. Do not cut the metal shelf this is illegal and is a structural part if the car. Most common stereo places will cut the shelf. If you need to then get roady it should not pass, then again alot of roady places do not know the law either.
3. It is far better if you install the speakers on a good fabricated rear shelf then on the original.
4. Do not try and display that you have a stereo otherwsie you may find that it gets stolen.
I have done this a number of time and my VR SS wears a pair of 8" Pioneers 3 way and a setup of 8" subs on teh rear shelf, (not metal removed.
If you want pics or understanding on how to go about this then post some more info.
When making for a VR/VS you will need to make a "skirt" so that the new parcel shelf is in the same fashion as the original, ie sloping at the rear "sixth" windows.
Cheers