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While trying to sort out why my left signal indicator was dull I also cleaned the selector switch. Put a large towel on the floor under the switch. The tiny machine-screw/nut and washer will bounce 20' if it hits a hard floor !
A Q-tip wet with lacquer-thinner cleaned the four terminals and the slide-rod. A tiny amount of di-electric silicone grease on these areas makes the switch glide, latch and manually unlatch, smoothly.

Hope you found it! If you didn't stop looking for a week and then you'll see it.........

Trying to find an electrical problem can be .....frustrating! As it finally turned out there were several issues with the turn signal indicator. Cleaning the bulb-socket terminals worked for a week and then half way through a short ride the indicator quit working entirely. Signals still worked ! End discovery: The grounding screw in the front left signal was loose enough to create the intermittent issue!

Scotch, you could completely disassemble the controls and clean them all. Mine were stuck and rusted, being exposed open to weather conditions for years. Nothing moved!!
There are little screws there which hold small plates to keep everything in place. Once removed (that can be tricky - those little screws might not want to go out), everything can be cleaned and lubricated. Easily done one afternoon. It's a detailed work, the kind I know you enjoy!

Everything good now. No need for surgery !

Good stuff!