My fuel gauge and temperature gauge are going nuts. Flapping all over the place.

I've been told that both of these units run on 3V that is delivered to them by a module that is located somewhere behind the instrument panel. It logically follows that the module is failing, since both gauges are failing together.

Does anyone know if a 300TD (wagon) has such a module, and if so where it is, how to get it out, and find a replacement?

Thanks
Gary