renault clio demister problem rear window?

I have a Renault Clio 1.2 dynamique 51 plate, the button for the rear window heater has stopped working and also the central locking button next to it, I have replaced fuses but to no avail! I had an after market stereo fitted, could this have caused the problem? I just cant find anyone online who has a simliar problem and i dont want to take it to somewhere for them to charge me hundreds for something that could be potentially cheap!