This mod is an add on to the extra
reverse light and under cab light mod. When all parts of this mod are completed
it will allow the reverse lights and the under cab lights to come on
when the truck is put in reverse or when the cargo light is on. You
can also gain control of this feature and be able to turn them on while driving
(Very NOT legal to do so please be nice to the tailgaters.) with the
stock cargo lamp switch if you do the cargo light auto off disable
mod. 
Before doing this mod (especially if you are not too electrically inclined) read over the How a relay works thread in the Garage and How a diode works on this site. To do this mod you MUST use relays to power your lights as described in the Garage section DO NOT attempt this mod if you just spliced into you lighting circuits to power your lights!!
Materials needed: wire cutters and strippers, electrical tape, wire nuts and butt connectors or solder iron and solder, depending how you want to connect the wires, any 1N4001 through 1N4004 diodes (you will need 2 of them) you can get them at Radio Shack for about .70 cents.
· Part A is to complete the extra reverse light mod as described in the Garage section under How to wire additional backup lights. I have an electrical diagram of this below:
The dots on the diagram are wire connection points.
·
Part B is to complete the under cab light
mod as described in the Garage section under How to install under cab lights.
I also have a diagram of this below: (A small suggestion, while running your
power wires to your lights run an extra wire to your back bumper and leave
it coiled under the bumper. It’s not any more trouble and this wire can be
used for something you might put in later. 
Like a 12v outlet to mount on your bumper.
The dots on the diagram are wire connection points.
· Part C is to complete the cargo light disable off mod in the Garage section under Cargo light auto disable mod.
At this point you can stop and you will have control of the under cab lights while the truck is moving through using your stock cargo lamp switch.
· Part D involves using the information on the diode that you read earlier. (You didn’t read it yet? That could get you in trouble.) Part D is described below:
Ok, now you have your extra reverse lights and under cab lights installed and have done the cargo light disable mod. Guess what? You already got the hard parts licked. Now you need to get back to your two relays you installed to control your extra reverse lights and under cab lights. Now find your trigger wires going to these two relays. Next find a good location work on the trigger wires.
1.
Now cut the trigger wire for your under cab lights and install the
diode in the forward bias position as shown in the pic below. Its important
that you have the diode facing the right way the cathode side (banded side)
facing toward the relay, if not your relay will not work. (See Examples 1
and 2 of How
a diode works if this is not clear to you.) 
2. You can either solder it in or use a butt connector. I soldered it so there is no chance of it coming loose. Now wrap the diode with electrical tape to keep it from touching anything or you can use a heat shrink tubing you can get from any hardware store in the electrical section for about $3.
3. Now repeat steps one and two except this time do it on the extra reverse light trigger wire going to the relay you installed for it.
4. Now between the diodes you just installed and your relay strip about a ¼ inch of the insulation off your cargo trigger wire and do the same for your reverse trigger and then solder a wire (jumper wire) between the two trigger wires and then tape with electrical tape, or you can use a 3m wire splicer, or you can just cut both trigger wires and use a wire nut to connect all four wires together. It is critical that whatever method you use to connect the trigger wires together that the connection of the trigger wires be done between the diodes and the relay!!(Once you let the smoke out of something you can’t put it back in.)
The black covering to the left of the wire nut are the diodes that have been heat shrinked. The power goes through the diodes to the wire nut and then to the relays.
Here is an electrical diagram of that the finished mod should look like.
Now you’re all done. Your reverse lights and the under cab lights will come on when the truck is put in reverse or when the cargo light is on and you can turn on your under cab and extra reverse light while your truck is in motion using your stock cargo light switch.
Acknowledgements: As much as I would like to I can’t take all the credit. I got the idea from one of Muddogs mods using diodes (if you want to get any info on electrical mods he’s the man to talk to) Special thanks go out to Tundra Soul for having such an awesome web site for all of us mod junkies to exchange information at, nhparrot and IZL for verifying the soundness of this mod before I tried it, and most importantly my wife for not getting too frustrated with the time I spent on my truck.
Do this mod at your own risk. I will not be held responsible for any damages done to your truck.
Courtesy of TS member Superman
