With a small circuit board and some diodes.. GOOP'd to the bottom of a light bulb housing.. or custom mount the whole thing.. Heat WILL run into an issue though

that is a possible way to do it, but I would suspect their life to be short if you did it that way.

circuit board, yes. Diodes aren't really necessary, a voltage regulator would be though. They would need to be mounted to a piece of metal that could act as a heat sink.

and w/o a heat sink, yes, heat will be an issue.

LEDs are sweet, but for the simplest cheap solution I took a regular reverse bulb, removed the glass and soldered in a 55 watt h3 bulb. cost me $1.50 each and theyre about 50% brighter. They cause a lot of heat but I havent had a problem with melting the lenses. Now if you wanna get creative you could put a tiny fog light housing inside the tail light behind the lens, that way youde have the fog light reflectors as well. Luxeons are kind of expensive for what you get. In my opinion.