Help with fitting aftermarket stereo needed

[Nuss]

Hiya new audi a4 owner needing help lol

I have a 2002 A4 fitted with a single din chorus cd player, now the problems are that ive come to fit my double din dvd player from my old car and cant get any aerial reception and also the rear speakers dont work anymore??

Ive read and read on forums but cant make sense of anything :(.
I understand that i need an aerial booster for the radio but dont understand about a wiring harness as the stereo plugs are the same on both units??

please help! :o:o

[Nuss]

Think ive cracked it :p

Ive bought a aerial booster and the lead to get the rear speakers working.
Quick Q tho, whats the wire for thats plugged in the center of the chorus unit inbetween the aerial and the power/speaker plugs??

[Nuss]

Just realised the cd player is actually a concert unit not a chorus!! So does anyone know what the little plug in the center of the unit is for??