You need to run two new speaker cables from the HU to the rear amp in the boot cubbyhole.
At the HU you will have a brown ISO speaker harness, this feeds the front speakers only and will have two pairs of empty sockets where the rear speaker feeds should be. You will have the male version of this plug on your new HU. You need to connect the new speaker cable to this connector (the empty sockets) so that the new HU will be powering both the fronts and the rears from it's internal amp.
If your new HU has this connector on it's own harness then you can simply connect the new cable to the HU harness (cables will be purple and green and clearly marked right rear +ve, right rear -ve, left rear +ve and left rear -ve) Ensure you match the +ve & -ve's at both ends of the new cable.
If the new HU has the connector on the back of it (no harness) then you'll need to buy a speaker ISO harness
Car Hi-fi Extension Cable : Wiring Accessories : Maplin Electronics
You can then plug the Audi front speakers into one end and the other into the HU. Then cut the 2 outer pairs on the adapter and connect the new speaker cable to the HU side of these obeying the +ve & -ve's as before.
In the boot locate the Audi harness feeding the rear amp. Disconnect the connector and either crimp/solder small blades onto the new speaker cable and plug them into the relevant locations on the connector or cut the feeds to the rear speakers and connect the new speaker cable to the loom side of the cut cable. This connects the new speaker cable to the rear speakers.
The diagram below is for the Avant, I found when I came to do my saloon I had to reverse the +ve's and -ves as the fronts were cancelling the bass on the rears. Just listen to the bass and if it seems bassier when fading to front or rear then you will need to do the same.