Buddy it depends on the hand work of the person and the Thickness of Wood.
I did the same thing, Do you want to know what is installed in my Vitz 2008 ??
6 speakers on the back wood. 2 Sony, 2 Pioneer, 2 Kenwood 718.
Under those speakers in my trunk Kenwood Sub Woofer.
Under the Kenwood Subwoofer. I changed the Ply which comes on the spare tire to a thick wood, And i screwed 2 Amplifiers to the wood. So the Amps wont slip light and left while i drive and if i need to remove my spare tire the Amps wont slip as well.
1 Capacitor screwed the wall of the left wood.
One Sony Bass Tube in the back seat Octangle shape face to the front wind screen.
And there is not a single sqeeqing sound while i drive, without music.
And when i turn my system ON, The car next to me on street Vibrates
So its the best idea to install the rear speakers, you dont need to screw or disturb the orignal body. And if the work is quality you wont feel or listen to a sound.