Are you sure someone didn't just remove the sensor in the radiator overflow and tuck the wiring?
Only the 87-89 GT's had the coolant light, but in 1990 i beleive both 5.0 models got it.
Half the 5.0's out there don't even have it anymore because the damn sensor sticks and causes the light to stay on, so a lot of guy unplug and remove it
