Adding a switch to Reverse Camera

[tfrezza]

Amateur alert!
I have a reverse camera installed in my van. At the moment, it's wired so that it draws power from the reverse light, so that when I go into reverse, it turns the camera on. All well and good, and it works. What I want to do is install a switch in the front of the van so that if I'm driving along, I can turn the camera on and see what's behind me. I have installed a switch and wired a constant 12v feed (stereo, will turn off with ignition) to it, then from the switch to the camera. The issue I have is that if I turn the switch on, the camera comes on along with the reverse light. Because of this, I think I need some diodes to control the flow so that the switch only turns the camera on and when it's off, the reverse light will power the camera instead. But I am unsure which diodes to get, as I believe they all different values. I know the following:

• Reverse Light: 12v 21w (I'm assuming 1.75a)
• 12v live feed to switch: (unsure of amps or watt)
• Camera: 12v 130ma

I know how to wire everything (see badly draw image) but I am unsure which diodes to get... can you help?
I think I need one 12v 1a diode (for the camera) and one 12v 2a diode (for the reverse lights), if these even exist!
Do I need to know the amps from the 12v feed?
Thanks

[clockworks]

Use a relay to switch the supply to the camera. Connect the reverse light supply to the coil, and 12v to the common contact, camera to the NO contact.
Connect the 12v supply from the dashboard switch direct to the camera (or the NO contact).