Ive posted this alot of times before, its the connection on the back of the block to the temp sensor- as sunny nutter says. Just give it a wiggle.
stnding from the front of the car its on the left hand rear side of the engine, under the inlet plenum, you can just about get to it by squeezing your hand through the gap between the plenum and brake master cylinder.
What you're looking for is a small casting, with 2 plugs/sensors stigging out the back. Just give the connectors a wiggle to clean of any grime, and it should solve your problem.
Also- nismoboy solved the problem by completely draining and refillig his cooling system- so if the connector doesn't solve it, then the latter should as i presume you had the turbo/rad off to get the manifold out.