The plug is already there, the amp has to be there as it is the same amp that operates the sub. If you look at the sub through the cubbyhole you should be able to see the red connector with a bunch of wires going into it. All the connections within the plug should be taken i.e. no spares, both going to the sub and away from the sub. If it isn't pre-wired for the rears (nice one Audi, saved themselves another 12p) then there should be more wires in the bottom section and less in the top. If it is pre-wired (matey at the dealer having a laugh), then the same amount of wires will be there top & bottom.
It would be really easy to connect them up even if they're not there. If you don't have a CD changer then you can access the wiring quite easily (it runs along a groove in the top of the sub) and should be able to pull it free and possibly unplug it from the sub without removing any trim. If you have a changer then you'll need to remove it first. Then get some cheap speaker cable and run it from the sub back to the rear speaker panel (which you'll need to remove at this point). the right rear can run under the back seats. The only tricky bit is wiring the new wires into the plug, I can send you the pin outs so you know which one goes where but I'd check to see if the cables are missing first.
