I have a 97 as well and it depends on if your car was an early production model or a late production model. Mine was one of the first off the assembly line and there is no temp sensor in the front bumper. One must have a temp sensor to get the reading.
You can turn this feature on by asking your dealer to enable the OBC (On Board Computer). if you hook a switch up to your dash, you can also get other things on that part of the display such as miles per gallon, miles until empty, etc.
http://www.whiteson.org/boxster/mods/obc/