Mine are not realistic, but I used the small, "grain of rice" LED lights and installed them from the back side of the body drilling a hole to mount.. Looks good when the engines are moving and not noticeable when idle. And the wires are hidden. try this link: http://www.modeltrainforum.com/showthread.php?t=302
From reading, I am not sure how the wires are exposed? They must be in the shell.
How about feeding two small bare wires against the shell and epoxy them to it, to insulate and hide.
The ultimate hidding I would think.
Model Railroader had an excellent article on the installation a few years back. They used an SD60 and a Dash 9. I use the technique they described and have had no problems. I do use a different light than they installed as mine is smaller. The results came out the same.