I'm not sure, but I'd say it would depend on your engine. No matter what, they'll be at the top somewhere.

There should be one where the hoses go in and out of the bulkhead (to the heater matrix) and another one somewhere near the block itself. Try just following the hoses from the radiator. I think that they usually look like the dust caps on a wheel but the one at the bulkhead might be different.

By the way, I had a 1998 306 1.9D that had similar symptoms. turned out that the head gasket had gone... See if there's any pressure in the coolant system. Or if the coolant has turned into a mayonaise sort of consistency. Or if there's any grey emulsion on the dipstick.

Good luck and sorry I couldn't be more helpful!