yes you can keep it. you need to connect the speaker outputs on the wiring harness to the rca outputs. you can search for LOC (line output converter) for more info on how to do it.

*edit: to make it easy on you:
https://www.rx8club.com/showthread.p...&highlight=LOC

 

Newest Metra kit. Should be available at any place that installs car audio. Should run about $150-$175. Then install any headunit, single or double din, you want.