Which system are we dealing with? Is the pump the mechanical style on driven by the wiper motor or is it an electric pump at the washer bottle?
Having just converted mine from the old mechanical style to the electric pump I've got some detail on how the stock system works. Well at least the 73-80 setup. However I don't think the switches changed much unless you have the intermittant setup.
The stock system has power to the washer solonoid as long as the wiper motor is on. The way the washer pump is engauged is by completing the ground circuit for the pump solonoid when the button is pressed in. On mine, the yellow wire to the pump solonoid was the power and the blue was the ground back to the switch. I tried installing my electric pump and still couldn't get it to work when hooked up this way. I then grounded the pump manually with the wipers on and the pump worked. Turns out my switch wouldn't provide the ground when the washer was activated. I swapped in a spare switch I had and everything started working again.
You have checked for voltage to the pump, did you check to see if it was getting the ground signal from the switch??? Here's a Schematic for the 73-80 setup:
