iPod classic battery has suddenly started draining in ~6 hours while turned off

Hi all...would love your advice!

I have a 6th or 7th gen iPod Classic (120gb) for 3 years now and it has been wonderful...until a few weeks ago. Whilst travelling overseas I started connecting my iPod to the stereo system (pioneer I think) of the rental car I was using, and all played fine, but after I disconnected it I and tried to use it the next day I discovered that the battery had drained to zero. I continued to use it for the next couple of weeks like this, each time trying something different after disconnecting to turn the battery off to see if it would not drain, but it always did...usually gradual and taking roughly 5 or 6 hours. (Normally I just turn it off by holding the play button). I played around with the Hold lock, but that change anything help.

So after coming home I hoped I would be able to get it back on my macbook with my iTunes with all the advice of support communities and get it working again. But there doesnt seem to be any recent topics on this with iPods which have been useful that I could find (some old ones where firmware updates resolved the problem). So...I have tried Apple Support's 5 steps to resolve generic problems, but to no avail. The software is apparently all up to date...both on my iTunes and my iPod (2.0.1). I restored the iPod and then replaced my music on it, then tried it again, but no luck. I then did it again and only put back on one album only (to see if perhaps it was a corrupt file triggering it) and tried again, but the problem persists. I have checked settings - backlight is set at 5 seconds and it has no problem going off. Alarm is not on. Nothing else seems to be unusual. I dont think it was ever was left in extreme conditions (certainly not recently, the weather was mild where I was). So I really cant figure it out. One thing that seemed to happen at the same time was that connecting to this car system seemed trigger the song shuffle function and it would always start playing the same random selection of songs. People talk about being able to hear the hard drive still spinning, but honestly I cant hear it while it is off or on.

So my best guess is that the stereo system triggered some kind of malfunction which makes the hard drive remain constantly spinning and therefore quickly drains the battery. But I have no idea what to do about it! Any ideas, I would be so very grateful! :-)