Hi another newbie intro !

Hi all been a member for a few years since I bought my first Disco in 2012 but just been a watcher and reader really. Anyway having loved the TD5 Metropolis for all that time I have just bought another one, an 04 TD5 ES this time. The main reason being I just couldn't get on with the Auto gearbox and all that goes with it so this one is a manual. Im just putting new number plates on it as the previous owner had private plates and found that Halfrauds had got the letters round the wrong way - genius! I have just emptied and dried out the back lights and the stereo has completely packed up, so just took that out to check fuses and power etc. Dead head, everything else fine. Ho hum ......! anyone know where to pick up a wiring harness adaptor for the Harman Kardon system that allows me to use an aftermarket Alpine head? ISO plugs all fit just not sure about the amps?? Also any good advice as to where to advertise the Metropolis ? Better sell it to pay for the inevitable mods and repairs coming!