On BMW sedans, it is common for the wires between the body and trunk lid to fatigue from the constant flexing. The wires either completely break in half (obvious) or break internally (wire insulation still intact). The symptoms are similar to what you are describing. I am not sure how the wiring is run in the E70 as I have not had mine that long. But, you should check all liftgate wiring for continuity (especially between the latch and fuse block). Check for continuity while manipulating the wiring by hand. I think you will find an internally broken wire or two somewhere.