A agree with the correct part number on the bulb, and make sure you clean the socket with something first. Simply replace the bulb in the 3rd brakelight now. Say hello to the black goo for me. (I had the wrong part number in my 93 when I bought it, and it caused the same problem. It looked identical and worked, but had a different part number).
I disagree with Pat'* advice about cleaning the contacts on the coolant sensor. The contacts being corroded would prevent the light from coming on, not cause it. The problem is a buildup of sludge on the face of the sensor typically. Inside the radiator. Clean THAT, if it doesn't help, either disconnect the sensor (will default to OFF on the light if you disconnect) or replace the sensor.