Try swapping the wires on the dual water inlet valve. Then try the water dispenser. The water should go to the ice maker. This will tell you weather or not the ice maker valve is working and if the line is clear.

The water inlet valve on the back of unit opens when the ice maker needs water. If the valve doesn't seal good because of sediment or wear it will continue to drip and when the water reaches the ice maker supply tube it will freeze. You can take the water line off the valve and watch to see if it drips.

A strainer before the inlet valve can also get stopped up restricting flow.