68 Convertible headlight knob

I honestly do not recall how the plastic switch bezel is held in the 68 model dash.
Some trip clock reset knobs will simply screw off. Other speedometers will let the stem free wheel counterclockwise so the knob wont screw off. If that is the case, you'll need to remove the 2 screws holding the speedometer in the housing and separate the housing from the bezel leaving only the speedometer in the bezel. Then you can hold that stem with needle nose, remove the knob, put speedometer back in housing where it belongs. At reassembly there is enough resistance in clockwise stem rotation that you can screw the knob back on. There are 2 rubber pieces, one on this stem and one between back of speedometer and housing ( Mentioned just so you know and don't loose/omit one ).
There are bezel nut specific tools for headlight switch, wiper switch, ignition switch, antennas, mirror controllers, etc.., etc... Most of us get by with other tool pushing the nut counterclockwise. Snap ring tool works in some cases. It helps to reach in behind and wiggle the switch body. They all have a square locating tang preventing rotation in the hole but you will find a tiny bit of movement there.