Some things you become so accustomed to in some cars that when you don't have it on another car, you really miss it. A trunk light is one of these things. A trunk light in a Mustang is almost an expected accessory and yet many Mustangs didn't have them.
   
The trunk light mounts in the square opening of the trunk lid underside just to the left of the center latch bracket.The trunk lights operate by means of a mercury switch inside the socket that turns on the light when the socket is moved to a certain angle. The power to the trunk light is always hot. If you find a trunk light from another Mustang or Ford from the 70's, the only things to remember are 1. get a light that mounts in a similar angle and side of the hole in the trunk lid as your Mustang, and 2. get as much of the wiring run as you can, preferably right up into the left 1/4 panel and the plug it connects to.
You can install a trunk light relatively easy. The main thing is to find a good condition trunk light and wiring harness. The "harness" actually is just one wire because the light socket itself grounds to the trunk lid to complete the circuit. You can find good trunk lid lights from many Ford cars of the era in salvage yards and they will all work. The original bulb is a smaller #63 bulb however you can use a #1156, as you see here, and the light is much brighter. The #1156 bulb is a good bulb for the hood lights as well. The trunk light bulb uses a single contact at the base.
The wire for the trunk light is fed through the trunk lid along the left inside. You can use the opening at this corner to help feed the wire through. It is then held in place along the trunk hinge and into the trunk.
The hot wire plugs into a female bullet connector on the main harness that runs along the inside of the driver's side 1/4 panel. Roughly at or just in front of the wheel well behind the rear interior panel, you can gain access to the connector if it is hard to reach from the trunk.
The connector end is red and the wire from the harness is a light green with a yellow stripe. Some later model lights have a square grey connector at the end of the trunk light wire. These you will have to cut the connector off and replace it with a male bullet terminal from an auto parts store. If your car didn't have a trunk light originally, then you will have to locate the other end of the green and yellow stripe wire at the dash. You will find this male bullet connector on a green with yellow stripe wire coming out of the main harness as the harness comes from a rectangular opening behind the dash and up about the level of where the padded dash meets the metal lower dash. From this connector, just make a wire with a female bullet connector on one end to connect to the feed wire and the other end with a flat spade terminal to plug into the fuse box under the dash to a terminal or some other 12v source that is always hot. Now you can always see your trunk contents easily at night.