i replaced my nak head because i wanted a cd changer and didn't need a cassette player. my old one works - at least it did when i had it removed. the rear amp had gone and the rear speakers were shot, too. i wound up getting a new head, rear amp and speakers.
You mentioned the rear amp. Does that mean there is more than one amp in the car? One for the back and maybe one in the front? Just wonder if the one next to the changer is powering all speakers.