See my avin install thread. You are using the backup lights to trigger the camera directly. Problem is that it does not always output +12v. It varies voltage for some reason. I found this out after probing and searching trying to figure out why it works with engine off but not on.

If you use the reverse wire to trigger a relay however, you can supply your own +12v and avoid problems. Or, like I did in my install, you can use a micro relay to trigger the camera when PDC is used. This is optimal as you can press the PDC when you want to see behind the vehicle.

__________________
2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)