My car came with a camera connected to a double-din radio and the backup cam comes on when I put it in reverse. If you don't want to go through a major mod like that, you can try what I used in my work van:
It's a front camera that mounts over top of your rear view mirror with a rear camera that mounts anywhere in the back. I chose to mount mine high so it'll work more like a rear-view mirror. It is great in my van because with the partition and all the equipment I have back there my rear view mirror was rendered useless.
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2002 Boxster Base - Arctic Silver - Tiptronic
2010 Subaru Forester
1980 Ford C-8000 Custom Cab Emergency-One Fire Truck
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"I never lose. I either win or I learn." -Nelson Mandela