Installing amps?
 

How to install new speakers/ subs and amps using the old headunit?

All you need to know is here : https://www.rx8club.com/series-i-interior-audio-electronics-24/speaker-sizes-harnesses-kits-etc-audio-install-reference-74664/
It helped me a lot.

This might help too:

https://www.rx8club.com/series-i-do-yourself-forum-73/diy-wiring-4-channel-loc-132087/