Maximum current draw from the reversing light circuit

Use the rear light wiring loom to drive a relay for a fused circuit that can deliver 500mA.

Generally relays are always the way to go with these sorts of applications. They take a small current draw but can switch significant current.

Here's an example on extremelights.co.za showing a relay being used for a much higher current draw: off road lights, but the principle is exactly the same.