On the e85 if you have a tv module, you can get a plug in lead which allows input of external AV sources or reversing camera.
Then just buy a camera from somewhere, aplug in and your away.
Only downside, is that you will have to switch from nav or radio to your tv tuner to view.
If anyone is interested, I just so happen to have one of the leads i mention above, and would be happy to sell for £30