Find your wiring diagram start here:
http://www.volvowiringdiagrams.com/?dir=volvo/S40%20V40%20V50%20Wiring%20Diagrams
No 2005 is there so I looked at the 2004 version you can look for the 2005.
TP3978202 S40 V50 2004 Wiring Diagrams.pdf

The layout is different than the older Volvo wiring diagrams, but you need to
take the time to understand the wealth of info in that manual.
The index and PDF pages are not the same but look around to understand how to
find what you want.
Index is on PDF pg 220
Components PDF pg 222
Component locations are just before the index and not adjacent to the
schematics

Go to PDF page 27 and 28 and look at the items serviced by the two grounds
31/66 and 31/67 (see pdf pg 214) see if there's more things dead such as seat
heaters. You added the power seat which helps understand a pack of things
quitting, The power seat shares the ground 31/66 with the L rear windows.

Just what grounds did you service?

The left pillar B is where I'd start
you can have a loose or corroded crimp connection.
If you suspect a bad connection from 31/66 back to the battery then you can
just hook a jumper cable from the battery negative terminal to the Left B
pillar and see if that fixes things, since there's more connections under
the hood that have to carry high current.

Bill