I did some research on other Volvo forums and discovered the following:

Beginning with model year 2005, the CEM design was changed to internal relays on the PC board (PCB) instead of external replaceable relays in sockets. My guess is Volvo made this change to save the cost of the sockets and make the CEM smaller.

The new CEM is evidently not a paragon of reliability. Many owners have complained of numerous electrical "gremlins". including at least 2 instances of one headlight failing. Problems are more common in northern states.

In Feb 2008, Volvo issued a TSB (techical service bulletin) to dealers describing CEM malfunctions caused by corrosion of the internal PCB and connectors. The corrosion is caused by a water leak (the CEM is mounted under the cowl area at the base of the windshield) and poor-fitting gasket. The solution is ($$$$$) CEM replacement, an improved cover and different gasket. This same TSB (or versions of it)seem to apply to some other 2005 models (V70 / XC70 / XC90 / S60). You can find reference to this TSB on the US government transportation web site (google NHTSA).

A TSB is not the same as a recall. It simply helps the Volvo technicians resolve a problem quicker. If a vehicle is out of warranty, the owner has to bear the cost (one reported paying more than $2000 for a new CEM).

If I still owned an S80 (now you know one reason why I do not), and it was out of warranty, I would probably increase the rating of the individual low-beam fuses and run a jumper wire between the left and right side. Alternatively, I might connect 2 diodes from the existing left & right low-beam wires to a relay, and then use the relay contacts to switch battery directly to the lamps. That way, I would only need 100mA from either of the corroded connections to turn on both headlights.