Only install HID on low beam, otherwise your ability to 'flash' your headlights will disappear due to the time it takes for them to fire! The bi-xenon headlights on later cars get around this by having a standard bulb purely for flashing, which also comes on in tandem with the xenons when running on main beam (bi-xenon effectively involves a flap that moves to raise or lower the beam, rather than switching 2 bulbs).
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