Backup lights as in illumination? Or as in reverse lights?
I also want to do the same thing, if you have the fuse # or where you tapped in, that would be great
The power to the back-up (reverse) light is used as a trigger.
When in reverse, the mirror is NOT dimmed.
There is no fuse that only feeds the Back-up lights.
It is easer to tap into the BLUE wire in the left kick panel.
This wire then runs down the left door sills to the lights.
It is very easy to find this wire.
Once you find what you believe to the correct wire, use a piercing probe to confirm.
Dave