Aftermarket radio in A3 sportback 05

Heya, this is my first post been hunting round the threads here and goin round in circles.
I want to put in my old pioneer head unit in my 2005 A3 sportback (facelift model complete grill)
The pioneers a lil more complicated than a normal single din as i have a hide away brain unit, which will fit in the boot. The brain unit has pretty much all the normal connections on it that would otherwise be up in the dash: amplified speaker outputs, preouts, blue remote power on, aerial connection... So I would normally have to run all of this upfront and make the connections.
My radio in the car is the symphony non bose so it gives an amplified signal to front speakers, and sends rear signal to the amplified sup which then goes to the rear speakers.
ok my dilemma /s is running cables back and forth when not required. Id like to just run front speaker connections up to the front and then tap into the rear speakers directly through the multiplug, then take the sub preout from the pioneer brain and connect that to the audi amp/sub (this will be replaced with a custom fibreglass enclosure at some point with seperate amp)

So wondering what adapter cables i would require, would this do Autoleads PC9-404 Audi Harness Adaptor (RCA, non-BOSE) £5.88 including VAT. PC9404. Buy discounted car audio online at In-Car Express
bearing in mind i wont be using the phono connections up front i need to do those connections in the boot. Im also aware that without buying a canbus ignition module i need to run a switched 12v ignition source which ill hopefully pull from the fusebox.

my other question is aerial related, i know i need to supply phantom power to the aerial amplifiers that is simple enough, but this car has two aerials the right and left, I understand the theory of what these do switching between stronger signals, but the audi HU does the switching, one connector is the ZF aerial which is terminated with a fakra connector, and the other is the HF aerial terminated in a normal din type connection. Do i need to join both these aerials together and then have it powered up, or disregard one aerial and just power that up, so far i havent been able to find any aerial amplifier adaptors which have both din and fakra connected, only dual fakra, or one or the other going to an iso aerial connection.

Any advice, tips would be much appreciated im hopin andymac might throw in his 2p as he does seem to be the guru here

wow, thanks for the quick response, currently the cable looms i made up for my old car, convert all or move all the connections upfront to a standard iso connector, so it is possible for me to have all connections upfront with the head unit, bar the pre outs, although seeing as i have to run cables from the boot to dash i could still just run a couple of phonos up there too. would the connector then work in that scenario? suppose the other option would be to disconnect the rear high power feeds from connectors and convert those into phonos and take the signal from the preouts? but in that way the front speakers would be louder than the rear

I get what your sayin about run another amp, thats the logical thing to do, but i used to run a 12" DVC sub on this system with a fairly hefty amp, i think the amp was rated 600watts rms if i remember rightly, dont really fancy running the power cable for that amp just yet and it seems overkill for that tiny lil speaker

...Fakra ignored= Fakra what fakra

Just looking at the pin out, have i got this right, pins 17,18,20,21 are the feeds to the rear door speakers, so if i connected my amplified rear signal to those the rear door speakers would work
then pins 1,2,3 are the preout signal into the factory amplifier. So i ?could? take my sub preout signal into pins 1,2,3 my blue remote power to pin 9. Is pin 23 a permanent 12v say goin to a fuse box or does it take the power from the factory HU same with the earth, is that earthed to HU or to a chasis point

I was just thinking, I have 6 preouts on the brain, fronts, rears and 2 sub. As the loom i made to run to the front has the 4 speaker lines, possibility of turning the rear speaker lines to a phono lead. But ive dismissed that as quick as i thought of it.

according to a questionable copy of elsa 17,20 should be positives. its then says 22 - Terminal 31, 23 - Terminal 30 dont mean squat to me
reason i asked about where they are connected is to use the audi amp, if i remove the factory HU will those two cables still be active, that way i can disconnect the multiplug as suggested, connect my high level rear signals to the rear doors, amp switch on from the pioneer brain, same for the sub preout, then connect pins 22,23 back to each other again?