Thanks for the vote of confidence, folks!
In order to add a set of rear speakers, you need to replace the rear door looms. This involves dismantling the rear doors almost completely, including taking the rear window frames out. So, if you're going to attempt this, be sure that you're aware of the realignment procedure when bolting the frames back into place.
In order to replace the rear door looms, you'll need to separate the connectors that go through the B-pillar. Be aware that this connector type changed during the A2's production run (between 2001MY and 2002MY), so the replacement looms must be of the same type. Also be aware that, in the later-style connector, the pin arrangement changed a few times between 2002 and 2005. This problem is easily remedied as long as you've got a set of terminal extraction tools.
You've then got to do the wiring on the inside of the car. You'll need to add the necessary pins to the inside half of the B-pillar connectors and run these into the driver's foot well, where you've mounted the Blaupunkt amp. The Blaupunkt amp will require a supply from the fuse box (Fuse 2 if my memory is to be believed) as well as a connection to earth. You'll then need to run 4 wires from the head unit to the amp: signal left, signal right, common earth and switched 12V. All these wires need to be pinned into a large black connector that fits into the Blaupunkt amp. Needless to say, you've got to be comfortable removing/refitting a fair bit of the interior.
The stereo will then need to be coded correctly, so VCDS is necessary.
If you're pulling the necessary parts from a breaker, life gets a little easier, but not much. You'll need a set of terminal extraction tools in order to free the loom from the miniISO connector behind the head unit. The loom's branches to the fuse box, earth and the rear speakers will need to be cut and then extended for re-installation into the recipient vehicle.
I've done this upgrade numerous times. I've also currently got a kit of necessary parts. So, if anybody wants this doing but thinks it might be a step too far for them, I'm happy to oblige.
Cheers,
Tom