Lighting Wiring

[R-N-R]

I would like to hook up a set of aftermarket backup lights connected to the factory reverse lights, but also switchable from the bed and/or the cab.

I also would like to hook up a cap light that is switchable from the bed and/or from the cab.

I prefer the bed and cab switching to be independent of the factory light switch.

Each of the above connections are reasonably simple, connecting them all together without burning anything up is what worries me. I also do not really understand what a relay does or when to use one.

I have read old posts but I was unable to comprehend any of the wiring explanations or diagrams that I found. I also did not find any posts with this type of triple combination.

Is anybody willing to explain this in simple terms?
I would greatly appreciate any help

Bill