This thread
http://www.mapleleafup.net/forums/sh...ad.php?t=28237 explains the operation of the Ford temp sender and gauge in the standard 6v system, and notes the different system in 12v setups. Perhaps your 12v conversion has used different components and resulted in a dysfunctional circuit?
The gauge operates by voltage heating a bi-metal conductor which deflects the needle. This should not be polarity dependant in a Positive or Negative earth change.