Dave,
Pumps, especially centrifugal pumps like a water pump do not like to suck so if your tank is in the front of your car and you have a long line to the pump inlet you either need to get the water level above the pump inlet so that you have a little positive pressure on the pump inlet, and or shorten the line length from the tank to the pump. Also increasing the line size, even bigger than the pump inlet size will help. From the description it sounds like your water tank is not pressurized, true? Mount the pump next to the tank it will work better. Pressurize the tank will also help the pump inlet. I also agree with Promachine check to make sure it is turning in the right direction, it is a DC motor so changing the leads will reverse it.
Also remember even if you get the pump to prime and actually move water if you have a bad inlet condition on the pump it will not pump as much water as it would if the inlet was optimum.
Rex