I have an '04 non-bose A4 - Is using the Blaupunkt adaptor simply plug and play or do you have to make some modification to existing wire harness? From what I've read, I would think the factory harness would at least be using the Rear outs for the amp in the trunk. If mods are required, what have people done - I've searched the forums but haven't found anything as concrete as I would like.

The Blaupunkt adapter is to get standard RCA's from the the OEM head unit, so you are by passing the Audi loom. If you want to use the standard loom with an aftermarket HU then you need the active speaker adapter.