Got a 2003 Allroad 2.5Tdi Auto. Earlier in the day I left the cd player on with ignition off. Last night battery/alternator light was on. Meter showed 12V and faded during the journey. I thought that it would re-charge during driving. Lights and all electrics died. Limped home. Tempperature guage OK. Would this be a battery failure or does it sound like alternator problems?