If your camera isn't wired to your reverse lights (i.e.: only powered when in reverse) you can split the cameras RCA cable with a Y adapter and run one to the backup camera input and one to the AV input, then use AVIN to view the camera at any time. Note AVIN will take over the sound as well as the picture, but this is the easiest way to do that AFAIK.
I have a problem with my camera - when the HU is fully booted up, it does not switch to reverse view. When it is booting (i.e. while in boot animation), it shows up correctly. I checked the signal, it's 12v when engaged reverse. I've tried reflashing stock rom with Malaysk, reflashing mcu; nothing works. Enabling the front camera in factory settings, putting in reverse and manually starting AVIN with reverse camera, it works. Any idea why it stopped automatic shift into "backup camera mode" and how to repair it?