You can bypass the rear amp and use the old wires. Not very difficult to do. Just remove the amp and splice the wires together.
No new wires needed, no re-wiring.
As for brakes, just because the light is on, does not mean your pads are worn, someone may have not put in new wires when doing the pad replacement last time. Mien were worn and I just spliced the wires together, as my pads did not have the proper attachment for them anyhow. This will turn off the light.
Check your pads though and if you bypass the wires, you will need to manual check them after you replace them.