I'll defer to Klaus and Phil but you should be able to "tap" into the positive on the headlight wiring harness.
You would have an always hot wire, that should be fused itself, coming from positive either on the post in the red box where you jump the H2 off or, to have a secondary fuse, on one of the posts in the fuse box that is beside the battery.
Run this positive to a relay and have the relay triggered by the positive at the headlight harness.
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