this will simplify what we're trying to say:
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to put things VERY simply in regards to our trucks....and most vehicles.
thin gauge wire(16G or less) is run from the "battery" to the headlight switch and then off the the headlights......there's lots of resistance in the various connections and long runs of wire.....in electric speak, resistance equates to heat....our trucks headlight system is already overtaxed with it's 110w of low beams and 130w of high beams. notice how when you switch on the highs the lows turn off? that is to aid in the truck not self immolating.
a lot of us have built or bought stand-alone harnesses to run our headlight system, using much better wire to carry the load. this also makes the lights BRIGHTER, with out column mounted switch/wiring being used as the "trigger" to turn on relays that in turn power the headlights.
any auxiliary lighting system should be on it's own circuit......multiple circuits CAN be used in conjunction with one another. there are various ways to use different switches, relays and such to make them operate the way you want them to.
the "Search Forums" tab in the upper left of this page can help in your quest for answers beyond our few replies..........
or if you have a few weeks/months/years and lots of coffee you could try and read a thread addressing these very issues......there's TONS of good information to be had in there, unfortunately it's mixed in with the "off topic" ravings of it's lunatic author, whom happens to be me.....
many good folks have joined in to provide their experience and knowledge on this very topic you've inquired about.
http://www.gmt400.com/threads/spyder-headlamps-my-take-on-em-and-lighting-in-general.36647/
at 90+ pages at this point i think a TABLE OF CONTENTS is going to be necessary, LMAO