Just to close the circle (in case someone else encounters this issue)...
The iPod went back to BestBuy two more times (the fine print on the extended warranty states they will repair or replace at their choise), the problem got escalated to their regional support manager, and was finally repaired correctly.
It was the broken first pin - the AVIC header is fine.
Thanks to everyone that contributed, empathised, or just flat out provided input!