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On roof etc gives better light coverages. Lower it is the more linear the beam. That helps show humps and valleys in the terrain. I like em higher. Few tips. You can trigger the relay so it turns on with high beams (Tap the highbeam light wire) thus eliminate the switch. Or not. Also use quality wiring harness switch. Some come with very cheap wires and they fail asap.. I run through fuse panel and use relays. IMany options and pretty easy. Interweb search may reveal many types brackets for placement.
I put two lights on my bumper and two on the roof of my Pioneer. The bumper lights don't do much at all compared to the roof lights. Put them all on the roof.