Thanks for the last comments above!
How to take out the parcel shelf seems to be a well kept secret. I took out the back seat and unscrewed everything I could see from above and in the trunk, even removed some of the interior pieces around the small triangular window, but still it was completely stuck. I've managed though to pry open a crack to take out the original tiny 8" subwoofer. Then it was quickly obvious that I couldn't reuse the original or BOSE bracket for my JBL 12", so I cut a double piece of plywood, in total 30mm, that I was able to bolt into the car and attach the JBL into.
Connected the cables and turned on the music and huh?. Honestly, if I didn't know what I've done, I wouldn't hear any difference! What?! How could this tiny 8", that looked more like mid/high driver if it wasn't for the actual size, perform like the 12" JBL?! Hm, I noticed that both the std and BOSE subs was firing downwards, could it be phase problem? Switched the cables and listened again. Still no difference! Did it need to be broken in? Scratched some gray hairs while wondering how could it be this massive upgrade with the doors/dash speakers and not the subwoofer...................????????
I know it's not ideal to mount a subwoofer in "free air" like in a parcel shelf or even towards the backseats, but this was unexpected. Was this as good as it could be in so-called "free air" installation? Did I need a dedicated box to get it better? Questions buzzed around in my head.
Half expecting that I needed to build a box for the sub, I decided to connect the amplifier mentioned earlier in the thread, that was planed to be used together with the sub first. Before I connected the amp, I noticed that everything was set to lowest from factory, so no loud sounds was expected when I turned on the music. Earlier I've set the sub level in the head unit to max to even get any decent response from the sub and now it was alive and kicking back there! Even at the sensitivity level in the amp set to minimum, the sub came to live as I've set the level in the head unit so high. Adjusted the level in the head unit back to normal and less than quarter of a turn up on the amp to compensate slightly, the output from the sub was more balanced with the rest. The I saw, as this is a dedicated subwoofer amp, that the LPF was set to minimum too (50 Hz)! Means it only let through frequencies lower than 50 Hz,and still it was kicking good! Adjusted the nob to about 80-90 Hz and lowered the level a bit more and now it blends quite good. Still lack a bit of definition on the bass as I can't hear the tones moving up and down from a bass guitar or other instruments with deeper tones. I'm pretty sure this would be much better with a dedicated box.
I like surprises, because then I can learn something, and this was completely unexpected! The thing about adding an amp to the subwoofer was initially a test to see if the new amp triggered the surveillance police in the car and shut down the music and throwing fault codes. It did not trigger anything but the sub to come to life, so in the light of resent knowledge, I've ordered a new four channel amp for the door/dash speakers. For now I'm awaiting how things will sound after this amp is installed before deciding if I go for a dedicated box for the sub too. Maybe I'm in for another surprise?
To be continued...