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Sounds like a bad connection to your headlights that is resolving itself when you activate another circuit. Look for a pinched wire or bad connection. Wire terminals are often at fault but look ok at a glance. Thin gauge stranded wire can break inside the insulation leaving only a strand or two to carry all the current. Take the plastic off the headlight and move the wire harnesses around with the engine running and see if the headlights get brighter. Use a good piece of wire between a wires terminals to test a wires integrity. Logic can not be your only tool with these oddball circuits because when you do find the problem you will most likely say "go figure".
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