I have done this mod to my mk4
get the fag lighter assembly from a mk3, this has a slightly different ring and a lamp fitted to it, then connect the lamp up to the traction control switch illumination wire, this will make it light up when you have your lights on or connect it to the supply to the fag lighter itself, this is not so good as the lamp will be on all the time
the fag lighter lamp illuminates the ash tray too