The sending unit is at the back of the engine on a 1.8L. "Coolant Temperature Sensor" is the correct name for it. Don't go asking the guys at AutoZone for a temp sending unit, 'cause they'll say they don't have one. They will, however, have a "coolant temperature sensor"! Mine was about $40 from them.
It is a three wire setup, and sends signals to both the computer and the temp gauge in the dash. I had the reverse problem on my Sport, the temp gauge would stay right in the middle, but the engine would boil fluid out. I had put an electric fan in place of the missing(when I got it) mechanical fan, and the aftermarket fan sending unit went bad. Anyway, I replaced the sensor, and now the gauge moves at least. I'm putting in a set of digital gauges to keep a closer eye on things though. As a plus, my mileage went up about 1 to 1 1/2 MPG!