As a new LS400 owner also thank you very much, this is the holy grail of LS400 stereo lighting repair, without I would never have found that dry joint. Now I just need to fix the LCD display which probably will need replaced.
Whilst I had mine open I also checked each bulb in turn by applying around 3.6 volts across each bulb, and noted which ones glowed.. Bulbs appear to be connected in series, so you probably can't do a continuity test. I found one bulb to be blown and replaced with an led soldered in place.
