I live on a Island called Mayotte where a pot. is hard to find ! ;-)
The post service are very slow (approx. 3 month to get my glcd from dx after shipping)
I've tried to connect the contrast to 0, 3.3V, 5V and differents values that I made with some resistances.
I can assure you the at 3.3V and 5V the contrast is very high.
I realy think that my problem is not here... :-(