1983 XJS. Only the passenger side brake light works. Replaced driver's side brake bulb; wiring going to the bulb looks fine. What could cause one brake light to come on besides a burnt bulb? Never encountered this on any other car before. Thanks

 

If one side does not work, test the incoming wire to the brake light fitting to see if it has 12v. If it does, clean up the fitting where the bulb goes and also clean up the earth return wire. If no 12v post again.

 

I don't know if what I say next came as equipment on an 83 but (I think) there will be a sensor tucked up in the rear wiring, the wiring of the light,,, you will/may find a small rectangular silverish box... It's the lights out warning sensor... I deleted mine. Lol, my girlfriend is the sensor now,,, she checks my brake lights every so often...

 

The brake light bulb failure module is most likely in the center console. I had a bad solder in that, that resulted in both brake lights not working.
In my case there was actually 12V at the bulb sockets when applying brake, but the voltage dropped to zero when loaded by the current draw from the bulbs, resulting in no brake light.
I would therefore as next step try to measure, if you have 12V present at the socket. If yes, then you're looking for a bad connection/solder

 

Solved: brake light failure sensor was surprisingly fine. But I noticed when removing the bulb that there was a little circular piece of metal sitting inside the socket unattached to anything (almost looks like a washer, but very small and thin). I centered it on the socket and reinstalled the bulb - brake lights work!