What exactly is the problem.

Can't you get any sound out of the rear speakers.

Or

Can't get the equaliser to stay in four channel mode

No sound out of the speakers could be car speific but equaliser not staying in four channel is unlikely to be car specific.

If you can get the old head unit in there even in a quick lash up for ten minutes could eliminate the empeg from the spotlight but only if the rear speakers are not doing anything.