To temporarily power and test components, you must only connect the ground (earth) to a suitable point on the chassis, then the 12-pin black connector to its mating connection mounted in the vehicle behind the audio amplifier.
In order to test the navigation computer and finalize your installation, you will require this:
navigation connecting lead: 61126907842 (this was fitted from 9/00 on, though one from any E39 9/97 on will suffice)
You will also need the speed and reverse signal harness, or you can make those connections on your own. Not needed for testing and not essential for function, but they are a part of a proper installation.
My memory's a bit foggy, but the connections you should have now on the main audio harness are as follows:
Two 12-pin white and blue connectors to the front screen
Four 8-pin black connectors to the joor jambs for the speakers
18-pin blue connector to the video module
2 TV antenna leads
12-pin black main power connector
Ground comb connector (to chassis)
White remote turn on lead
3 main connections to amplifier
12-pin connector to subwoofer assembly
Power and signal connections to the CD changer
Main radio connector
White 6-pin connector to the navigation lead needed from above, which in turn will have the other (white) 18-pin connector to the video
module, the two connectors for the navigation computer, and the black 6-pin connection to connect the factory speed/reverse signal harness