passat battery problem

I am having a problem wiht my 2002 Passat battery dying overnight with no forwarning. I brought it to VW, charged me $100 and said it was my cell phone. Being an electrical engineer, I looked into it myself and found that my cell phone does not draw anywhere near enough power to cause a problem. The analysis done by VW was inaccurate in that they were assessing the draw when the dome lights were on. Shortly afterward the radio stopped working. My guess is that the radio amplifier is not being shut off as it was causing the power draw. Now that the radio stopped working, I think the problem may be gone.