As above, my understanding is abysmal of using a multimeter, but I'm learning. I think it's just got to be logical, take it steady, and isolate/deduct one thing at a time.
I had a fuse blow on the Central Locking circuit day before our holidays, perfect timing. We'd recently replaced most of the doors, so anything could have happened. Rany my mechanic mate and save faffing with multimeters and so on, his nice, simple approach was 'get a handful of fuses, disconnect/reconnect the doors one by one, soon as the fuse stops blowing, you've found the door.' each day out we did just that, by the 3rd day found the faulty door. CL solenoid at fault.
If you're having inconclusive results from the multimeter, try a similar approach? Not entirely sure what systems you can safely remove a fuse from (well, theoretically all of them, otherwise what happens when a fuse blows?) yes, the car might not start/windows stop working etc but at least you've found/discounted your fault each time.