Disco 2 Fitting aftermarket radio for hi-line system

Hi guys. I have been reading many posts about fitting an aftermarket radio into Discovery 2 but couldn't find the clear solution/answer.

Is there anyone here who has done it in hi-line audio system to have everything properly working?

I have 12 speaker system with sub in boot, cd-changer (don't need it if that's a problem), steering wheel buttons, amp under passenger seat and audio remote controllers with headphone option for seats in boot. Everything was run by alpine cassette radio player which is gone now, so want to replace it with some modern unit to have usb etc.

Thanks a lot!!!