a3 sound help

The yellow is the front & rear preouts
The blue is the CD changer bus (if fitted)
The green is CANBUS which is unused on pre 99 A3's
Unfortunately the rear pre-out is used for both the tupperware sub and the rear speakers. They all run off the puny amp inside the tupperware box. The stock HU has no rear amp inside so if you want to run a proper sub off the rear pre-outs you need either a 4ch amp (2ch's for the rears and one bridged for the new sub) or you need to keep the stock amp in the circuit. You can buy the BLAU RCA adapter which replaces the yellow section of the multicoloured plug. You can the run proper RCA's & remote on to the boot, however without a 4ch amp you will then lose the rears.
Easiest solution is to just unplug the red sub connector and pick up all the feeds you need from that plug (saves running all new cables), I can supply an adapter loom which plugs into the Red block without needing to cut any wires.
But you still have the issue of the rears. The other option is to leave the rear amp plugged in and use piggyback's to tap the 4 wires you need for the new sub amp. The pin out looks like this below, and you only need left, right, screen and remote on. If you go this route I would disconnect the tupperware sub speaker as it will interfere with the sound, but ensure you tape off the wires as they will be live.

No probblem. Yes that's probably the easiest option. Good Luck, let me know if you need any further help.