Depends on what brand & model your HU and changer has. Usually pag same brand plug ang play. But there are some HU models that doesn't have aux in plugs.
Alam ko not all batch from the same brands apply, tkae for instance Alpine HU. meron ainet ready ang bagong units while other CD changers are not and they are not compatible even if the same brand.
Usually alam yan ng store kung san mo bibilhin ang CD changer, kung magkatugma sila o hindi.
kung masipag ka pwede mong tanggalin ang mga scuff plates para maangat mo ang carpet para dun itago ang wires (if u plan to place it under fromt passenger seat)