Troubleshooting: I started at the CEM fuse panel, and tested fuses 84 (25A), 85 (25A), and 86 (5A). All 3 fuses had 12v from the positive side of the fuse to a grounding point, meaning they all had power up to the fuse. This indicated to me that the circuit was open somewhere after the fuse, and that the relay (2/89) that’s built into the CEM is likely good. So next I unbolted the driver seat to tip it back, and tested the wires at the connector (64/30). Pin 14 had no voltage to ground, so the fault is between the connector and the fuse panel. I then traced the wire back and found the next connection point (below).
The Problem: Branching Point 63/173. There are 4 wires that are all connected at this junction, as seen in the diagram and picture below. I would assume this wires are twisted together and then covered in some sort of tape. However due to its age and my V50’s tendency to turn into an aquarium this connection corroded and completely disintegrated. This branching point is found under the drivers seat underneath the carpet, maybe 4″-6″ from where the branch of wiring for the drivers seat comes off. You can also find it by tracing the wire from pin 14 of the connector under the seat. The branching point was on the underside of the harness, which is likely why it corroded so badly.
The Fix: In order to fix this issue, I needed to reconnect the 4 wires (obviously). However due to the wires being corroded off up to or slightly past the sheathing, I needed to add some length. What I elected to do was use a waterproof butt connector and a short length of wire to extend the single wire that runs to the rear cargo area light. Then I stripped the ends of the 3 wires that go to the drivers seat, passenger seat, and Fuse 86. I twisted all of these together, and then added some solder to ensure a good connection. I then shoved the twisted wires into another butt connecter, and attached them to the other side of the wire I had added. I also put heat shrink over the two waterproof butt connectors, so hopefully I never have to deal with this again. The power seats now work along with the rear cargo light.
It appears that most of Volvo’s branching points are done in a similar method to the one that failed, so unfortunately I expect corrosion at the wiring junctions to be more of an issue as my car ages (and fills itself with water). On the bright side, once diagnosed the repair only took ~2 hours with removing the carpets, and I now know what to look out for if more electrical gremlins appear.