That generator light bulb definitely must be isolated from ground. If you can't find the right socket and don't want to modify the instrument cluster housing then you may want to consider a bright white LED. Radio Shack sells these things and I can give you a number if you can't find it. The LED will need to be wired in series with a dropping resistor, say 680 ohms. LED's must also be wired with the right polarity. You can then put this LED into the "tube" that likely exists and hold it in place, maybe with a dab of glue from a hot glue gun. May take a little effort to replace but the LED will likely last a LOT longer than a regular bulb.