You could do it all under the hood with relays. Sorry but I don't have time for a diagram just now. I believe that the stock headlight switch already switches the positive connection to the headlights.
I'm am struggling with this... I can't get this to work. I have terminal 86 running to a switch. I have a wire running from the high beam to the switch.
If I connect to one wire of the high beams, the aux lights are always triggered (assuming the aux switch is on). If I connect to the other side, the relay triggers when the high beams aren't on (aux lights would be on) and when I turn the high beams on, the relay turns off.
If I disconnect what I've done, both sides of the high beam plug go directly to ground, so that's why, the aux relay triggers when the lights are off. I would think that since it's ground switched, the high beams would not go to ground until the switch is turned on.
I thought about this some more and retested my work...
Now, I'm at the stage, that if the lights are off, I have to manually turn off the aux lights. But, if the lights are on, then the aux lights work as intended, off when the low beams are on, one when the high beams are on.
So not perfect, but I guess I will be ok with it. Any other ideas?