Andy, the car's an A4 Cabriolet B6, 2004 reg. (non-Bose).
A few years back you described the installation of a relay to provide the switched 12v supply for an aftermarket HU. I really like the solution & made up the relay on a piece of veraboard in a 'project box' ready to wire in. I then went looking in the car for what you described as the 'very tiny red wire' running in the plastic trunking to the left of the steering column. Undid the trunking & amongst the looms found one thick red(disregarded) & one medium red (about 2mm dia. including insulation). Is the medium red, the one you refer to? It's not really tiny - not as fine as the grey & blue running alongside which are <1mm.
In case Andy doesn't see this, can anyone else who has done Andy's mod. successfully advise. I don't want to cut the wrong wire.
Thanks for any help, B