This is fairly easy, but I figured I'd document the procedure for anyone who hasn't worked with sealed lights before, so here you go. I'm assuming you've removed the fog light assembly from the vehicle.
What you need:
Tools - 2 flathead screwdriver (small, large), heat gun/oven, c-clamps, allen wrench, scissors, heat-resistant gloves
Broken fog light lens/housing
Replacement part for the broken lens or housing
3M Windo-Weld Ribbon Sealer (I used 08610, .25" x 15'. ~$20 at your local auto parts store)
Optional - ~3" rubber o-ring or gasket, HB4/9006 bulb, green scrubby pad
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Disassemble the fog light:
Remove the bulb by twisting counter-clockwise
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Remove the bulb socket from the rest of the fog light assembly by slightly loosening the white hex adjustment screw enough to swing it back and to the side, and popping the bulb housing out of the side retaining clips.
Remove lens from fog light housing:
Heat the lens and housing, either with a heat gun or by placing it in a preheated oven at 250 degrees for ~15-20 minutes.
Once everything is warmed up, the adhesive/sealant that holds these pieces together should have softened significantly. *Wear gloves*, everything is certainly uncomfortably warm to the touch at this point.
Use the smaller flathead screwdriver to separate the lens and housing enough to slip the larger screwdriver in the gap. Work your way around the seam with the two screwdrivers and you should hear the adhesive breaking free. *Work all the way around* before attempting to remove the lens to avoid damaging anything. Things get brittle after a decade under a car.
Clean *everything*:
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This is a good opportunity to make sure there's no dust, grease, fingerprints, etc. Clean everything up. Additionally, you're going to want to get all of the old adhesive/sealant off the part you intend to keep. For removing from the lens I found my fingernail and a green scrubby pad worked better than anything. From the housing, you'll need to do some scraping with a screwdriver or other tool. Try to get it all, it will help seal as well as possible and increase longevity.
Reassembly:
It helps to work on a cutting board or other clean surface for this part. Cut off ~1ft of the 3M Windo-Weld from the roll.
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If necessary, roll the Windo-Weld a bit thinner, and work it into the gap where the fog light housing meets the glass lens. Press it in evenly all the way around.
*Remember to clip the gray inner section of the housing back in now, before replacing the lens*
Gently evenly press the lens back into the housing. Place everything back in the preheated oven for another 15 minutes.
Wear gloves again (duh) and tighten c-clamps around the perimeter of the lens/housing to ensure complete and even seating. Remember this stuff can be brittle. Leave it to cool.
Remove the c-clamps. If you're adding a gasket/o-ring, place it around the bulb housing now, and clip it back into place with the retainer clips and hex adjustment arm/screw. Install new bulb.
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You're done. Well, once you put it back on the car anyways, but you got it off, right?
Let me know if I missed anything....