I never did like the earlier fox body tail light lenses like my car had. They looked like chopped down Fairmont ones, and they were hard to keep clean.
 Newer Mustangs at the wrecking yards always had smashed lights, or were missing the lenses. So when I finally saw a good pair I grabbbed them.( Those wreckers charge $1.00 an inch for tail lights. Ouch.)
 The rear panel of the car needs different holes. The proper way would be to cut off the rear panel and weld in a new one. However, the way I did it works too.
 The wiring harness is different so make sure to take it as well. The old lights shared a bulb between stop and tail lights. The newer lights have separate bulbs, which also have different plugs.
 It is also neccessary to rewire the brake switch. Cut the bottom wire on the brake pedal switch and run a new wire all the way to the rear harness. I had a piece of red wire just the right length.

  The new lenses seemed pretty dirty inside so I tried cleaning them in the shower. However the only good way was to take the lights apart and scrub them. The lenses are held on by a series of tiny metal clips that point the wrong way. It takes patience but the lense can be pried off.
 While they were apart I wondered why auto reflectors are painted flat silvery gray? Ceiling lights and most other lights are gloss white. They must be mimicking the pot metal that housings used to be made of.
  Anyway, I painted mine white. Later, I saw several other websites where they did the same, or applied chrome tape to the insides.
  I peeled out the old rubber and siliconed the channels, then snapped the lenses into place.

   I brought some cardboard and a feltpen while I was at the wreckers to mark out the holes. The new lenses need different holes in the metal tail of the car. The outside hole is okay, but the others need to be cut out. The middle bolt holes also change from up and down to side by side.
   I applied the pattern with a white felt pen and used a holesaw and a jigsaw to cut out the openings.

 Here's a view from the inside. A piece of sheet metal was bent over and spot welded in place to hold one of the new studs. One of the old stud mounts was cut out with the hole saw as it was in the way of the lights.
 The old holes need to be covered to prevent exhaust and noise from entering. Fiberglass and resin was laid on a sheet of glass and peeled off when almost dry. It was cut to shape, glued on with more resin and held in place with the tail light. I suppose if you used more layers you could use that to mount the studs without the welding.

Where did all that sand come from?

 The holes were recut in the fiberglass, then sanded and repainted with a spray can blue. The color is off but who's gonna see it?
 Now repeat everything as necessary.

 I really liked the newer lights. The whole look was updated with this one change. TR