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Which of the following are you talking about? "Cab light" isn't very specific.-roof marker lights (above windshield)-cargo light (outside above back window for bed)-interior dome lamp (inside middle above back window)
Ok. Now which interior pieces do you have?
Ok. That's good. Now as far as I know, but I could be wrong, all you will have to do is drill the holes in your cab for all the screws that are used to hold the pieces in place. It sounds like you got the dome lamp itself, did you get the wiring for it also?
New wires and switches are good, nothing wrong with that. Which relay are you trying to find values for, what system is the relay for? What year, make, model, trim is your truck and what year, make, model, trim was the donor truck?
It does not use a relay. What year is your truck and trim level?
We aren't sure what your asking for? You want to hook up an interior dome lamp, your asking for a diagram. Diagram of what? Wiring diagrams? Diagrams of how the dome light mounts to the cab? You say it's going to be custom wiring, what exactly does that mean? Do you have some kind of setup or idea you want to use? The factory system is simple, works well and pretty easy to fix or trouble shoot if it stops working. Your intent here seems to be vague and we don't have enough information to help you affectively. We still don't know what year your truck is, what trim level it is? All of this information is important to know to figure how to help you the best we can. Your going to have to explain in detail exactly what your plans or ideas are, what pieces you have and/or need and any information you have about the truck that can help us help you figure it out.