Could be someone polished the car with pledge, dont laugh the stuff works! and sprayed into the scuttle air intake area OR has clear wax oiled the scuttle to reduce rusting, mind you wax oil stinks of paraffin.
OR polished the interior though this wouldnt explain why it occurs when you use the A/C
Try one of those there impressive A/C bombs that kill 99% of all know smelly germs