Audi *****s breed like rabbits
The answer is above, but I had a similar issue until I realised the blue/white rear amp remote wire has to be connected to a switched 12v supply, to receive the signal to power up the rear speakers.
Your new head unit should have a separate wire, often blue, labelled 'remote' or 'ant' that will supply the signal required. Note that 'ant' wires are primarily intended for aerial amplifiers, which you may need if radio reception is poor, so the head unit may only supply power to this wire while using the radio. You may get lucky and it stays on when the CD etc are used. If not, you need to find a switched live to use and preferably run a new one from the fuse panel itself, to avoid overloading an existing circuit.
One final point, check that all black wires in the line out converter adapter you used actually have a corresponding black/earth connection on your original Audi loom. If not, your rear speakers still won't work even if the blue/white wire is correctly connected.
Noticed others mention RCAs, your AUDI loom doesn't have them judging by the pictures.
Last edited by Allroader; 3 Weeks Ago at 10:44.
Reason: RCA issue