I have had my 2013 Mustang for about 3 months now, and put 3,000 miles on it. I have really enjoyed the car, but one thing that is lacking is the trunk lighting. The trunk is lit by a single festoon lamp at the very rear of the car, low to the trunk floor. The light is very easily blocked by most anything. A popular mod is a LED bar/strip attached to the roof of the trunk, but I have not found a very good how-to, so I decided to write my own.
The LED strip I used is from SuperBrightLEDs.com. I got the “cool white” 40 inch version. (WFLS-CW60-WHT)
Rear Trim Panel
To get to the wiring, you have to remove the rear plastic trim panel. It is held in place by six plastic fasteners, two of which are hidden under the top of the panel on either side of the latch hook. These are a little tricky, you just have to get close to one and pull up, after you get one pulled out, you can slide the panel towards the other side of the car, and the fastener will slide out of the panel.
Trunk Light Connector
Here is the trunk light connector, the light is ground switched, with the switch being in the trunk latch. The yellow/green stripe wire is the positive wire, and gets turned off with the battery saver relay circuit. The white/blue stripe wire is the ground wire coming from the switch in the trunk latch.
I spliced the positive and negative wires from the LED strip close to the connector. I also used some heat shrink to make the job look professional. Here is a tip, push the trunk latch closed on the trunk lid to kill the ground going to the connector. That will make it less likely of a short happening, but you still want to be careful of the yellow/green striped wire, as it will still be hot-unless you wait for the battery saver relay to shut the power off. If you really want to make sure, just unhook the negative side of the battery.
Here is a link to the service manual with the wiring diagram for the trunk light. 2011 Ford Service Manual — on the second page.