Swapping 96-98 tails into a 94-95 is about the easiest modification you can make to a Mustang.
Tails will run you about $100-150 a pair from eBay if they include the 96-98 harnesses, a little cheaper if you get them without, and a little more expensive if you get a set from a salvage yard like MPS.
The 96-98 taillight harnesses are necessary to install the 96-98 taillights.
Remove the plastic screws holding the trunk trim panel behind the taillights. Once you lift these, you should see four nut-and-washers on long bolts holding the taillights in place. Unscrew these - I used a 7/16 wrench to get them loose and then just removed them by hand.
Next, unplug the rear taillight wiring harnesses from the main body wiring. You'll then have to squeeze the rubber grommet out through the hole in the body panel to remove the taillight.
Finally, make sure the 96-98 taillight wiring harnesses are connected properly to the 96-98 taillights, and reverse my previous instructions to install.
Piece of cake! Swapping them out shouldn't take more than 30 minutes, if you have all the tools and parts needed.