Front mount/rear mount...sounds a bit like mating season on the Discovery channel.
A rear visor mount is when you take your visor, usually bigger than the T-visor area, and insert the visor from inside the helmet. In other words, it's pressing the piece of plastic behind the visor area, which gives the illusion of a perfectly cut/shaped visor.
This gives the visor a nice recessed look similar to the film helmet. In fact, the film helmet had a rear mounted visor that screwed into place. This method usually works well for standard Dp's from the '96 and up. Not that you couldn't use it on a '95, but it already has a recessed area on the front of the helmet that the others don't have.
A front mount is when you take your visor, cut to the exact dimensions of the T-visor area, and you lay it down on the T-visor area from the front/outside of the helmet. This method is best for the Dp 95's, as it as a recessed area that nicely accomodates a visor placed from the front. I can't speak for how well it works for the later models.
Hope that clears it up a bit.
Edit- Obi Wan's Jedi Academy has an interior mount tut here:
http://www.jedi-academy.com/fett_visor.html
And if you check in the general information section there is post with Fett links:
http://forum.thedentedhelmet.com/viewtopic.php?topic=122&forum=1
There are couple sites in there have visor replacement info.