Unfortunately my electronics knowledge was even more limited at that time and I did not realize the tape head was biased at ~12V so I connected the ground pin of the line-in directly to the common/ground of the tape head. This caused my iPod to reset each time I tried charging it with the jack phone connected.
A few months later I switched to a 12V power hub which included a really crappy USB port as well. The moment I tried to charge the iPod while playing smoke came out of both the supply and iPod.
In hindsight this was a stupid idea since the ground voltage of the charger was at 0V and the ground voltage of the ipod (headphone and usb) was sitting at 12V (it should have been 6V but the cassette player had a boost supply).
Bought a broken ipod classic 6g with a 160gb harddrive for about 10E. The 'new' ipod was not even booting, the 1.5" harddrive was clicking all the time and a sad face was over the screen at all times. Sold as-is, obviously, seemed to (barely) have survived a car crash.
In a strange change of policy, Apple has decided that ipod harddrives are interchangeable, so I was able to transplant the old 20GB harddrive into the new 6G ipod without any electrical changes. There is some mechanical play seeming that the 160GB HDD was a bit bigger, but two restores later my new ipod with old HDD was up and running.
No pictures, everything is just plug and play if you want to try this. The hardest part is opening up the case without leaving any marks (not possible I suppose).