I'd recommend modifying a fuse so that you can measure the current through each circuit. A lot easier and saves a lot of running to the trunk. This is what I did.
to do this break the fuse material, then solder a wire to each term on the outside of the fuse. You can then connect multimeter to the fuse and measure current through the circuit.
An e39 is like a beautiful wife, needs constant attention, lots of money, lets you down at the worse times but you’d never be without it.