To avoid running the increased current through the wire harness, use the same circuit at the selector switch, not at the light end of the wires. And the high beam indicator will illuminate when the high beam wire is active. no need for adding anything.

But the big question is how much "safety"does it gain for you by blinding the oncoming traffic???

The newer version of the lights are meant to have both beams on simultaneously when 'HIGH' is selected. I'm not trying to do anything unorthodox with the lights, just make them work fully in an older model of the car.

 

So it sounds like, one wire/terminal of the three is a constant 12V feed to both filaments (H and L).
Then the other two terminals are switched to GND/CHASSIS to light filament.

In old car, H and L were never on simultaneously.
In new car, H AND L are on (switched to GND) when HIGH beams on.
I would assume (can you verify) that the WATTAGE of the H filament has been reduced in the new model bulb such that:
Wattage(H.old) = Wattage(H.new) + Wattage(L.new)
If that's the case there is no need to worry there is increased CURRENT on the HIGH beam switch/relay as the intent is to still output the same WATTAGE, just now using two filaments.

So a diode connected between H terminal (cathode of diode) and L terminal (anode) should also turn on L filament when H filament is on (switched to GND/CHASSIS).

 

In other words, use Tonyr1084 diagram in Post #19 BUT:
Change COMMON- to COMMON_12V
Reverse diode polarity
Change HB+ to HB- and LB+ to LB-

 

I've realised that my car will pass the annual MOT test with the single-bulb aspect, so I'm going to leave this just now & come back to it.
Thanks all for trying to help.
Gary