It is located in the cast aluminum housing to the left of the motor (viewed from the front) at the end of the upper radiator hose. It is hard to remove. Special "E-clip" or "Snap Ring" pliers are the ticket to getting one out with a minimum of cursing.
Check out http://944-world.com for some general 944 info.
Check the 944 FAQ at http://www.connact.com/~kgross/FAQ/944faq.html. Part 7 covers the cooling system.
Hope this helps!
Bill
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