Tail Light Conversion

hmmm.... a little too broad there. There are several type of conversions out there. From body conversions (usually found with cadillac conversions), OEM conversions (most common; just replace with aftermarkets) and there are lens conversions (uncommon, but most commonly found with import conversions especially Si conversions).

Is there a particular type you had in mind? Or particular vehicles? It makes a HUGE difference on what techniques you will need to make a sucessful (and street legal in many places) conversion. 

Body conversions will use metal fabrication work 95% of the time. The other 5% will use fiberglass (4%), polyurethane (.8%) and carbon fiber (.2%) depending on the car and other factors.

Clean lens conversions will need a plastic welder of some type and a light flame to blend and weld the plastic lens' smoothly.

A fe wmore details plase, so we can offer the appropriate advice.

"Ditto" for Bellsracer.  If your wanting to integrate a different tail light set or headlamps in the place/or a different place, from a different vehicle that is in a metal area of your vehicle, fabricate it with metal....Truckin/Minitruckin magazine have ads for the Cadillac conversion.  All you have to do is fill in the factory light hole with metal, and cut metal out to weld in the Caddy's prefab light housing, and assemble the rest.  Finish with required body work.  Didnt one of the truck books have a snap-shot "How-To" last year?  If your wanting to do this in a "Plastic" part of your car.....you already read the drill above...

Anyways, like Bellsracer requested, can you provide more detail to your question?

If you purchase a Caddy light kit from somewhere like FBIRIDES.com, airbagparts.com, or other.  These kits come with the hardwares and prewired lighting pods.  Now, if you are taking a tail light from another vehicle, try to cut the metal that the tail light is mounted to and remove it.  Use this part with the light.  Weld it in and install your light as if it were ment to be there.  As far as wiring, if your 12 volt challenged, educate yourself with a book meant for your car and adapt the lights wiring, its not that hard.  Now, worse case, lets say you have the light only.  If youve paid attention enough to the forums in here about fiberglassing, cover the back of the light with foil and make a replica to have something to mount to and fabricate the FG to the body and finish out the body work, then wire it up. 

Did I misunderstand something?  Hope I provided enough detail, ask if i missed something.