What I do know, is that troubleshooting a fuel gauge issue is way easier in the Roadster - the connector on the top of the tank is exposed when you raise the lid to lower/raise the top.

Non-functioning fuel gauges are rare but we've seen a few in the thirteen years I've been here. It's always been the sending unit in the tank or the connector atop the tank (in Roadsters, as water can get in there quite easily in roadsters). In the "Wiring" section of the service manual, go to "Instrument Cluster" and you can see the wiring. It's pretty simple, there are two wires to the level sensor and the resistance of the sensor varies as to fuel level.