Be interested to hear how you get on Danny, I was going to go down this route but couldn't get a clear answer anywhere on whether it would work or not.
It's all a little bit muddy, and here's why:
Fact 1. The HiFi coding means the output is now a balanced signal over the existing speaker wires.
Fact 2. The speaker wires in the car are a pair: + and -
Fact 3. In terms of audio, a balanced signal requires 3 wires: +, - and ground.
Those 3 things are irrefutable, but they don't agree. I can only assume that the low level balanced output from the HU is just a + and - signal, rather than a signal and ground which is what you'd get in an unbalanced setup for 'normal' RCAs.
Anyway - the point I was trying to make is that I don't know whether you can use both speaker wires to solder to an RCA plug, or whether you'll have to use one wire for the tip and find a ground for the ring.
Just how many Yorkshire sheep can you fit inside one exhaust?