Thanks for this information - really appreciate it.

Any idea which wire colors belong to the liftgate?

I looked at my X5 liftgate yesterday and it has the typical wiring in rubber boot between body and liftgate (or body and trunk lid on sedans). You will need to disconnect all the wiring in the liftgate and pull the wiring harness out of the rubber boot far enough to inspect/repair the wiring. The problem will then be obvious and most likely right where the wiring enters the rubber boot from the body. Simply splice the wires back together and then, this is the important part, attempt to reposition the portion of the wiring harness that receives the most flexing upon reassembly. Either more of the wiring toward the liftgate or more toward the body. The important part here is to change where the wiring is flexed each time the liftgate is opened/closed.