Trueno headlight issue ...

[Ivan141]

I think the fuses are always on the 12v side of the circuit, usually not too far from the battery, so that's the wrong place to look..

So lets assume the bulb is getting power, then all that is needed to switch on the low beam is a connection to ground though the switch. The high beam indicator is wired after the low beam coil, parallel to the switch.
When you power the lights on, the headlight relay closes and both coils of the bulb get 12v. This can only happen if you turned on the low or high beam.
When the high beam is grounded through the switch, current can flow and the bulb will light up. The low beam coil also sees 12v but will not light up because no large current can flow. However, the little current that does go trough powers the high beam indicator in the dash.
Now when you turn on the low beam, the switch grounds that side of the circuit, and the high beam indicator is effectively grounded on both sides so it should turn off, whilst the low beam coil now has a good path to ground and current can flow.

What this means in your case: Your high beam indicator is working, this means current is getting through at least one low beam coil. Your high beam and flash work, these go to the same ground as the low beam.
I'm inclined to think something is wrong with the switch.