I have a 2000 SSEi and only one fan turns on when the car is hot. If I am not moving the car will get hot during 100 degree days and the warning light has come on a few times, but once I get moving the temp will down to 200.
The setting has to be changed in the PCM with a tuner. That applies to the pre 2000 bonnevilles as well. The 180* thermostat will have the coolant flowing, but without adjusting the fans they will not come on which means unless your driving you have no air flow through the radiator.