the "bestest" way is to run one relay to ground switch another relay so you can run a switch to the dash that also turns on your aux reverse lights
(and some people are going "whut?"
A relay as you know is an electromagnetic switch allowing you to switch larger amounts of power with a very small amount. This way you can run a light amp wire to the switch and not have to worry about frying a switch.
But a relay does not care what it is switching
it can switch power OR ground, it just completes that circuit when its electromagnet is powered.
Now this comes into play when wanting multiple switches for 1 item, like reverse lights. A switch to go on in reverse (trans) and a switch on the dash to turn them on anytime. But if you run 2 POS switches to something it will fry something in the process when both are "on" at the same time. But you can multiple ground with no issues. This is how the interior lights in older cars work, the door switch "grounds" and the light comes on.
Lights get constant power, no switch on POS just fuse.
So you run the factory reverse lights to switch one relay, to complete the ground for the lights.
You wire another relay to be switched via switch, to complete the ground for the lights.
So now you can be in reverse (one switch) and have the dash switch on (2 switch) and nothing gets fried as all you have is a better ground.