I would suggest using something like a 21W lamp with flying leads to check the wiring. You need something that will draw some power (voltage plus current, not voltage alone) from the system. Modern multimeters draw a minute amount and are not particularly useful for finding bad connections. They’re perfect for finding broken connections though. To use a water analogy the water pressure (voltage) in your home is essentially the same as the pressure in the Main outside but flows (current) less because of the restriction of the smaller bore piping, equivalent to a bad connection in vehicle wiring.
Faultfinding really is just checking in a methodical way, start at the battery and work away connection by connection. Don’t try to check both positive and negative at the same time i.e when checking the positive line leave one of the wires connected to the negative terminal whist you check the connections on the positive side and vice versa. A bad connection will cause the lamp to be less bright. And of course if the lamp doesn’t light up well when put across the battery then you’ve got a really simple solution, recharge or replace.