Check for power at bulb connector. If you have battery voltage then check your ground for oxidation or an open circuit and repair. If power and ground are good then the bulb needs to be replaced. If no power remove the fuse and visually inspect if you say voltage is fluctuating? If any doubts about the fuse then replace it. Attaching a wiring diagram as well. You should always have battery voltage at the fuse. It is better to check the fuse using a test light. The test light should either stay lit to show it is good or it wont lite up. That been said, you should also check the wire from the light bulb connector to the central electronic module connector for an open. The brilliant engineers at Volvo figured it would be nice to have the light relays built into the CEM for some reason. They never gave it any thought as to how anyone could replace the relay? I guess they figured this way they would never fail? But they do fail. And once they fail you need to replace the central electronic module. The good thing is that on this model it is a plug and play unit. The bad is that when you need to replace a control module then they are expensive. If you check the fuse with a test light and it is good but you have no power at the connector. Check the wire for an open and if that is good the control module has failed. And I have seen the CEM fail not just for light issues but also wipers because. YUP they built the wiper relay inside as well which you can not service?
On the diagram 10/84 is the drivers headlight. 10/7 is the low beam bulb. Once the relay closes contacts it delivers the power from fuse 21 in the engine compartment. The pin number is F4 from the CEM. If you had an issue with the fuse then check the wire from CEM F3 to the fuse for oxidation. If you had a short the fuse should have popped already. And as you can see from the diagram on the other side of the fuse the power comes from the +30 battery cable. If that was an issue then you would have all the fuses going 0-8 volts like you mentioned earlier. That is why I say just check it with a test light. So let us know what you find please. Also the CEM is behinde the sound proof panel on the drivers side. You need to remove that panel out the way to get access to the CEM.