I recently acquired a 1974 series 3. The switch on the left top in photo does not work. When I toggle it, it feels loose as if it is not connected to anything. However, when I toggle the headlight switch next to it to the middle or on position, the dashboard lights come on, illuminating the gauges. Is it possible that a previous owner wired it so that the interior lights just run through the headlight switch so that the interior lighting switch is redundant? It is a soft top so there is no dome light. It is a military version and I do know it was rewired from 24 V to 12 V.

 

Mine was a 2A so not necessarily the same. IIRC my dash lights had a switch of its own, not through the headlight switch. Do you have the wiring diagram? It's a very simple system and won't be difficult to trace.