I cracked open the Bentley repair manual and have used it to diagnose and temporarily fix my interior light problem - they weren't working, but all the fuses and relays were good, as were the lamps.
I've tracked down and diagnosed the problem for the interior lights. Everything is fine with the exception of the Interior Light Switch (OEM Part 9559766). I tried to fix the switch, but the corrosion/oxidation on the inside was beyond repair. Someone at some point had installed the switch so it was only connected to two jumpers. I figured this might be the problem, but no such luck. I've ordered a new part from an online vendor (don't know if I'm allowed to mention them by name, so I won't, since there might be fees for advertising here and we don't want to cheese off the pays) and hope this should do the trick.
I'll keep you posted.