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Of course, after thinking about it, I have figured out how to do it without even a single diode.

1) Use SPDT or 5 terminal Bosch relays.

2) On both sides, current headlight signal wires to 86, ground 85.

3) On both sides, connect the light to terminal 30, and the fused 12V to terminal 87.

4) Connect a jumper from the high terminal 30 to the low terminal 87a.

5) This will take a 35A to 40A fuse on the high beam side - but what that does, is if the low beams are off, the lamps themselves are wired with the high beams. If, however, you turn the low beams ON, then the LOW beams will be lit.

This also means that a failure of one side is isolated from the other side.

Only possible problem is if either low beam bulb shorts, it'll blow BOTH fuses as you try low then high beams. I'd consider using circuit breakers instead of fuses for that reason ...

But not a single diode to be seen anywhere!

So MadMikeyL's worry is taken care of, MY concern is taken care of, and the OP gets to cause his/her night vision to degrade.

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