Any of us that have ever driven at night with cargo in the bed have wished we could turn the cargo light on without the interior lights. After much research, I took on the task of wiring my cargo lights to a switch. It worked, and I love it. As an avid duck hunter, this time of year requires me to be up early in the morning and carrying valuable cargo. It's nice to be able to actually check on it while I am heading down the road. I created this write up to help others do what is actually quite simple. If you have any questions, please ask and I will try to answer them.
You will need a couple different colors of wire and probably 25 feet of each. You will also need a 5-prong relay. The use of the relay will allow the cargo light to function like it was from the factory (on when doors open and on when interior light wheel scrolled all the way up) and also work on the switch. You will also need a switch. Other than that, basic tools is all that is required.
1. Remove the cargo light by unscrewing the two phillips head screws. This is a picture after I had wired up the switch, but you get the idea.
2. Remove the interior dome light above the rear seats. This is for crew cabs. Not sure what the interior of other trucks looks like, so you will need to adjust as necessary if you do not own a crew cab. This just allows for easier access to get the wiring through and such.
3. I used a 5-prong relay and a lighted switch, so here is what I did. I ran two wires from the cargo light, to the front of the truck. The wiring for the cargo light runs to the interior dome light. From the dome light, it runs to the driver's above the rear seat along the head liner. In order to get the wires fished through, you need to first remove the foot plate from the driver's side back door.
4. This allows you to be able to remove the rubber door seal. You need to remove the rubber door seal past the upper right hand corner if you are looking at the door from the outside in. The start and end for the rubber door seal on my truck was located down at the foot plate. I am sure that is probably the case with most of these trucks.
5. This will allow you to pull the upper section of the plastic the covers the rear seat belt in towards the interior of the truck which will give you access to the headliner and a way to route the wires. After removing the door seal, just grab onto the plastic and gently pull in towards the truck. This will release the top attachment point, which is all you need.
6. Next, I used a fish tape from the side door, ran it between the headliner and the roof, and aimed it towards the rear cargo light. Once it came through, I attached my two wires, and pulled them back through. Once you have them, you can route them down the seat belt pillar that you popped off and down to the foot plate that you removed earlier.
7. Remove the drivers foot plate, and use the fish tape to pull the wires towards the front of the truck.
8. You can now route the wires through the plastic molding on the left past the e-brake pedal.
9. I used a powered switch that had three connectors on the back. One for power in, one for power out, and one for ground. I grabbed power from the horn fuse and ran it to the power in of the switch. The horn has constant power, so you do not have to have the keys to turn on the light. I then connected the red wire I ran from the cargo light to the power out of the switch. The white wire I ran from the cargo light was connect to the ground and I also ran another wire from the ground terminal of the switch to one of the screws back behind the brake pedal and used that for ground. This gives me a hot wire (red) and a ground wire (white) running to the cargo light. Here's the switch.
The hard part is over. Now it's just a matter of tying into the existing cargo light wire and attaching the relay.
10. The wire that runs from the interior dome and powers the cargo lights is a black wire with a blue stripe, seen here.
11. You need to cut this wire. This is where the 5-prong relay comes in. In the interest of full disclosure, I got the relay wiring instructions from post #11 on this thread but adjusted for the use of a lighted switch.
Toggle Switch for Cargo Lights - PowerStrokeArmy
Here's what the thread says for the relay wiring (I changed some of the wording to try to make it easier):
Cut the wire to the light (the black with blue stripe wire) and run the one from the cargo light to 30.
The one from the interior light to 87A.
Run a +12 (red wire) to 87.
Loop 87 to 86.
Ground one side of the switch and run the other side to 85 (white wire).
Relays come with 87A and 30 closed. So it will work like normal.
When you flip the switch it will close 87 and 30.
This is what your wiring will look like.
YOU'RE DONE! It's now time to test your switch. Once you confirm everything works to your liking, you can work on butting everything up and enjoying your switched cargo light that still works like from the factory. This job took me a couple of hours to do and I am just a regular Joe. I've learned a lot of things from this forum and gained a lot of knowledge. This is my way of giving back.