While what PalmettoDisco linked to will work, you are still comprimising the the intergrety of the wires because you are cutting into them. The crimps that come with that adaptor cut into the wires to make the connection.
http://www.roverparts.com/Parts/9439A.cfmPlugs in line with the wiring, unplug the harness from the tail light, plug in this harness, and then plug this harness into the tail light.
That simple, literally 10 min install. This is the only type of convertor that I would use, if you can find it cheaper more power to you.

 

Thanks for the info, you guys are extremely valuable resource, I'm thinking the $70 just because even though I could probably do it, I could see it becoming very frustrating endeavor.

 

Keep in mind that the $70 one is Disco I, the Disco II is $100 (has a rectangular ISO plug, instead of the older round style).

The ScotchLocks do nick the wire, however, the metal blade has a greater electrical capacity than the original wire, and the electrical integrity is not hampered at all IF installed permanently. If you remove the ScotchLock, you will have a damaged wire remaining. The ScotchLocks are simple to install, however, on a Disco II the wires on the trailer harness connector are snug and a little bit of a hassle to isolate and install, once a paticular color is found.