I have a 2007 Ducato Maxi 120 Multijet and I am looking to add a rear-view camera and taking the power from the number plate light but I am wondering what is the positive (+) and negative (-) wire colors so I get the correct ones for the camera
Thanks

Not a good idea.
Your van has a "chassis computer" that monitors current consumption. Adding a load may trigger the light bulb warning light.
Buy a camera with a cable set (power and video) and connect it to the radio wiring or the 12V socket near the ashtray. The LCD monitor can be powered from the same source.

The obvious problem is not the computer but more that the camera is only going to get power when you have the lights on........

Thanks for your replies