Rear speaker replacement help - can't mount the speakers

[kiwi]

Hello,

I have NISSAN PULSAR 2002 Sedan (LX N16).

I have installed the rear speakers and is working fine, however, I can't mount the speakers and make it secure because:
1) I don't know remove the dashboard. I am able to get the seats out, but the dashboard seems to still be connected somewhere.

2) The car speaker screw hole doesn't seem to fit my 6.5 inch speaker and is not standardized.

Any help to mount the speaker, including suggestions on alternatives ways of securing it, is appreciated!
Thanks.