Got the multimeter out today, constant 12v on yellow, nothing on red with ignition on or off. I didn't think it was the fuse as the factory renault HU worked fine, however, checked the fuse and sure enough, it was blown. Swapped it out and still nothing, so I was like WTF? Checked it again and its blown again. Turns out the cigarette lighter has a small piece of metal in the bottom, that was shorting the fuse. Removed that and hey presto. It all works.