If you just want some music in the car & aren't worried about quality of install or where the music is coming from then 6x9's in the parcel shelf will be ok. However just consider the size of hole you have to cut out of the parcel shelf metal, the 5" speakers will fit in better & should you choose to change them later wont leave gapping big holes you'll need to fill.
If you want 6x9's (& as already said) & you dont care where the sound is coming from then buy/build a speaker board for under the rear seat.
If you want a more quality sound from your system then build speaker pods for your doors & mount some 6.5" components.
Yeah, it's not the fact that i dont want quality. Its the fact i cannot afford it, this is why i am just looking at one set of 6x9's on the shelf. Would i be able to get away with not cutting the metal, if I was to buy some spacers or something?
Cheers