Usually on the right side in engine bay. Two vertical metal cylinders about 3/4" in diameter and side by side. When one is sticking you get differing temperatures out of the left and right sides of the CC unit. Sometimes just a tap on the top with the handle of a screwdriver is a enough to release it. If not, try opening and cleaning or replacing. Most sites have a DIY since it's a common problem. Especially in cars where the coolant isn't being changed regularly.