Their is another tread similar to this but it shows it wiring the cargo lights to the stock dome light. This show how to successfully wire the 3rd brake light to your mustang dome light so that when you unlock, open the door, or anytime your dome light is on, the cargo lights will come on.
This is done on a 2003 Ranger
Philips Head Screw Driver
Here is what we will be switching out, stock red one to super duty or another year ranger's brake light with cargo lights
Simply unscrew the dome light and the 3rd brake light. Unplug the brake light. First your going to using the wire hanger to feed a piece of wire from the dome light opening to the opening of the 3rd brake light
Some say we need to use this harness that comes with the new brake light for this to work, but we do not need to use this
Using your wire cutters, cut the blk/pink wire that goes from the brake light to the plug simply leaving enough wire to work with
Now using your adapters, attach your new wire that you ran through the cab to the blk/pink wire you just cut.
I like to use electrical tape just to make sure its on their good and tight
Now using your mustang dome light, attach it to this screw using one of your adapters. it has to connect just light this because this crew is for the 'switched' power to the dome light. it fits perfect in there and goes right out the provided hole
And now we get to plug your brake light in and re install that. now reinstall your mustang dome light and push the remaining wire up in the headliner. after all is installed we can now test it to make sure it all work. it should look like this:
Now the door is open:
Now the door is closed and the dome light has shut off:
So when you unlock it and lock it, it should look like this:
And their you have it. The succesfull install of a 3rd brake light with the cargo light functional.