I last did my reflectors with aluminum foil in 2001. This left taillamp housing was actually pretty good because the seal was in good shape and the housing cover and wire grommets, accessible in the engine compartment, were all in good condition snugly assembled.
The right side had sucked up exhaust and discolored the reverse lamp housing and eaten dust from travels into stream beds and desert paths, and the right side marker was actually sporting a cup of water inside of it.
Here is the right side taillamp housing after the new foil application:
And here you can see the gain in illumination from fresh clean foil on the right side turn indicator versus the as-yet-untouched left side:
Remove all lenses and seals that are cooperative and wash with soap and water and a toothbrush for all the textured places. Rinse thoroughly and dry. I used a length of vacuum hose that just fit over the screws as a source of home made rubber washers, just cut every 2-3 mm and install one per screw as you insert them into the lens:
Remove all bulbs and wipe clean. Clean bulb contact surfaces with a little scrape and application of DeOx gel. Clean the reflector housings with soap and water and a toothbrush, dry with a wash cloth. Rinsing is less critical than drying since we are going to apply a light mist of spray adhesive on all the surfaces we want to cover with aluminum foil. Snag a roll of Reynolds Wrap from the kitchen. The foil is ductile and easy to work with around corners and across shapes. You can flatten and smooth pretty successfully as long as you do not apply tearing force. If you accidentally rip off a chunk, just get a patch piece from the roll, and spray a light mist on the back side of your foil patch and apply it to the bare spot. The smoother you get the foil to lie down, the nicer the reflectivity.
Side marker after foil application:
Be loyal to the shapes and cut-outs of the taillamp and turn signal housings, they are part of how they get the lens to breath so condensation does not occur. Last step is to make sure the foil is clear of the bulb holes, clean the sides where the bulb grounds, and clean the contacts in the holes where adhesive may have compromised conductivity. GumOut on a q-tip is good for cleaning ears in a pinch, but here it is good for cleaning adhesive off the contacts. Install the bulbs and wipe perfectly clean then don't touch. I use a drop of superglue to attach the ends of the seals to make a nice continuous circle and install with no adhesive around the perimeter:
Even in daylight, the lenses now have a nice new car luminescence, at night, even more:
To prevent cracking the lenses, you want to apply good firm generalized force to the lens as you cinch down the screws lightly and evenly. Don't let the screws dig into the plastic lens. You push the lens and let the screw take up the slack, just enough to get the perimeter to seal. No need to overtighten.
Colin
be easier than using foil with spray glue?
