I can not help you in the selection of a new CD changer, but guess Pioneer should have some new generation units able to interface to the old 14 pin DIN connector, may be using an extra adapter. What I know is that you should not bypass anything there as the "another box located on the right hand side of the trunk" it is the amplifier and the radio itself. Head unit only contains Cassette player and control unit. Whatever you do, you should keep "connection" between head unit and right trunk box.