I suggest you use earplug type speakers...
1) No installation and can not be seen (the engine will provide the Subwoofer
)
2) If you use the new "Closed" type with silicon they also funktion as ear protection (needed on my car
) - but then you can not hear sirenes ect.
3) I suggest to not install a fixed system - use a portable (smartphone...), and if you need a stereo when parked... buy a B&W Zeppelin Air or Harman Kardon Go+Play II and put it in the trunk - among the best sound you can get...
Just my 2 cent.
Have fun