My 740 wagon has no tailgate functionality...

no wiper function, no lock function, nothing...

Where do I start diagnosing this problem? I assume it's a wiring harness and not both motors having failed. Also, I cannot operate the lock with my key either.

 

The lock key operation is not electrical, so that's apart but the rest you just have to see if power is getting through the routing by the hinge. First check fuses, then take the gate apart (remove the cover). In some areas with rust these gates and assoc. accessories get pretty nasty. It may be a matter of just cleaning and lubing and tightening loose bolts....