Yeah i wouldnt bother with componants in the rear... if your gunna spend the money for them they should be in the front and you should mount them with some attention to either buying kick pannels or glassing them in...
As for the deck harness yes you need either that or the relocation harness if you wish to retain rap(retained acc power) the relocation harness is a pain.. i know there are some others out there that will mimic the relocation of thestock h/u without doing so... HOWEVER
you stated that you have an aftermarket single Din HU installed allready? if this is so someone has taken the time to both Remove/relocate the stock hu, and fit the aftermarket deck mounting kit that you need to fit a single din unit into a din and a half hole...
Because this has been done you should be able to cut the harness on the deck side of the joins and follow the wire color code to wire in your new alpine... usually these harness are the same...you can check this usually by a diagram that most manufactures place on the deck via a sticker. if that fails check online.. but 97percent of the time nowadays they should be the same...
Oh and dont forget to swap the cages (the part that mounts the deck to the mounting kit) as these are hu manufacuter specific.
Oh and Btw this
http://www.bcae1.com/ helps if your in a bind...