Never heard of a water neck. Can you describe roughly where it is on the engine?
That's the purge valve for the charcoal canister (although, as other posts have stated, for some reason Toyota calls it the "bi-metallic switching valve") -- lets air through once the coolant gets to a certain temperature, no air through when it's colder. OEM part number 90925-05068 . $80 is obscene -- MSRP is roughly $40 (just bought one from my stealership in the last month, and they always charge list, no more, no less). Should be able to find aftermarket for $20 or less...just search for the OEM part number.The first one's it.