I recently "adjusted" my headlights. My question is what does the adjustment do, aim the lights up or down forward or backward? It may seem like a silly question but, I never really understood how that actually worked.

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The lamps don't move. The adjuster tilts the reflector assembly up and down. Both the high and low beams are adjusted simultaneously.

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Bose Luxury Sound System w/Touch Screen Navigation, Addition of Factory XM, 2005 MY Antenna, OnStar Upgrade (3G),
RainSense Wipers, Backup Camera, '00 Style Door Panel Courtesy Lights, Heated Washer Solvent, 2X Remote Trunk Release,
Turn Signal Mirrors, Center Console Courtesy Lamp, Rear Outboard Heated Seats, PVD Chrome 18" Factory Rims, Upgraded
Carbon Fibre Appearance Interior Trim, Highly Modified Main Body Harness, Instrument Panel, Door, Door Panel & Headliner
Wiring Harnesses, Custom Fuse Box & Tire and Loading Information Decals, Additional Acoustic Insulation[/size][/color]

Thanks, so I thought maybe the bulb moved up/down forward/backward. So, the reflector moves up and down.

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Correct. The reflector is one large assembly for both bulbs which is why the low and high adjust together. This reflector is on pivot points which permit it to move when the adjusting screw is turned. Also, there is no horizontal adjustment screw for these. They are not intended to require a horizontal adjustment, assuming the front end (body) of the vehicle is not damaged or otherwise horribly out of alignment.

The bulb can move slightly because the base is attached to a rubber mount that, in essence, plugs onto the back of the headlamp housing. This is not designed to be adjustable. My personal belief is that it's intended to be somewhat of an insulator/shock mount to improve the life of the bulb by protecting it a little from sharp jarring.

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Bose Luxury Sound System w/Touch Screen Navigation, Addition of Factory XM, 2005 MY Antenna, OnStar Upgrade (3G),
RainSense Wipers, Backup Camera, '00 Style Door Panel Courtesy Lights, Heated Washer Solvent, 2X Remote Trunk Release,
Turn Signal Mirrors, Center Console Courtesy Lamp, Rear Outboard Heated Seats, PVD Chrome 18" Factory Rims, Upgraded
Carbon Fibre Appearance Interior Trim, Highly Modified Main Body Harness, Instrument Panel, Door, Door Panel & Headliner
Wiring Harnesses, Custom Fuse Box & Tire and Loading Information Decals, Additional Acoustic Insulation[/size][/color]