2004 A3 Replace Head Unit

The speakers and the power are on standard ISO connectors (or at least they are on older models).
You may or may not have a remote ignition on lead on the power ISO. Later HU's use the CAN-BUS to send a digital signal to the Concert to tell it the keys are in the ignition, so have ommitted the Yellow/red stripe cable that should be present 2 pins down from the permanent live.
If this is the case on yours, you will need to run a switched ignition on cable from the fuse box, fuse labelled Radio (S-Kont), to the HU to avoid draining your battery.
The Concert does use Mini ISO connectors for the pre-outs (to the rear speakers/sub) and the CD changer (if you have one). This is where you would need an adapter.
However, my recommendation would be to run additional speaker cable to the rear speakers and by pass the puny Audi amp.
Not sure if the new A3 uses exactly the same configuration as the old A3, but the Concert does not have rear speaker out, so I would imagine it is very similar.
You shouldn't need to run separate power from the battery (unless installing an amp).
Cheers, Andy

just got mine after it been on back order for a while.. perfect fit... now just need the HU

which is on order.. if anyone wants pics let me know and i will sort something out!