Ok, I know there are a few guides and posts in various places on the forum about wiring up a subwoofer to the stereo system in the Fiesta Mk7/7.5. 
Some of the posts do mention the RCA adaptor to convert high level output to low level input for a sub. None of the guides/posts, however, actually contain an install done using an RCA adaptor. 
Has anyone actually used one in a fiesta Mk7.5?

Secondly, the adaptor I purchased has 5 wires. Only 4 of them are described on the packaging (2x front left and 2x front right) but there's a 5th wire with no explanation as to what it does. 
Anyone know what that 5th wire is? I'm guessing possibly ground?

Lastly, I'm unsure of the colour coding for the stereo loom so don't know which ones are speakers and which ones are the right ones to tap into.
Would anyone be able to shed any light on this and help me wire the speaker outputs correctly by identifying which wires to use?

Many thanks. 
Nick

So, can anyone actually tell me which are the speaker wires I need to tap into?
Further info for you, it's a sync 1.1 system with the colour screen

The quadlock should be the same across Fords as far as I know.  Hopefully this helps:

https://www.fordwiki.co.uk/index.php?title=Quadlock_Stereo_Connector_-_Sony_DAB_/_USB

Thanks Tom. I shall go out tomorrow and test it all out. 
Do you have any ideas as to what the 5th black wire could be on the RCA adaptor?

rca amp & sub line out low level converter guide

LH rear speaker grey/white - terminal C442/8 from rear of stereo White/violet + terminal C442/4 from rear of stereo RH rear speaker Grey/white - terminal C442/5 from rear of stereo White/violet + terminal C442/1 from rear of stereo Both sets of cable are solid grey and solid white colours at rear of stereo however they change to there striped colours on the door card looms For best results on bass id splice in to the 6" component on tge door card itself, Because at that point the sound will have been split via the crossover box ensuring only the deep tones are sent to the 6" speaker, This way you will be sure its only deep tones going to your sub, Also dont forget about the 12v supply behind the drivers side boot panel, Run that to the amp save yourself running a feed from the battery as its already factory fitted. Your amp will also have an ignition positive sensor feed, This single cable generaly blue in colour needs to be spliced to a feed that becomes positive when the key is turned, just the same as the stereo does, Splice in to the large orange cable on the rear of the heater contro

Just now, yunii said:

rca amp & sub line out low level converter guide

LH rear speaker grey/white - terminal C442/8 from rear of stereo White/violet + terminal C442/4 from rear of stereo RH rear speaker Grey/white - terminal C442/5 from rear of stereo White/violet + terminal C442/1 from rear of stereo Both sets of cable are solid grey and solid white colours at rear of stereo however they change to there striped colours on the door card looms For best results on bass id splice in to the 6" component on tge door card itself, Because at that point the sound will have been split via the crossover box ensuring only the deep tones are sent to the 6" speaker, This way you will be sure its only deep tones going to your sub, Also dont forget about the 12v supply behind the drivers side boot panel, Run that to the amp save yourself running a feed from the battery as its already factory fitted. Your amp will also have an ignition positive sensor feed, This single cable generaly blue in colour needs to be spliced to a feed that becomes positive when the key is turned, just the same as the stereo does, Splice in to the large orange cable on the rear of the heater control

plesse note this was on a focus mk2.5 titanium with sony dab  unit ,as i did this . just compare with your car s loom. this is only a guide not instructions . so do research before going shesd .

yunii