Grab ahold of the light switch under the dash you should feel on the side of the light switch a spring button. Push the button in poll the light switch knob out. Then you unscrew the nut that holds the switch in. Once you get the switch out play around with it so you can see how it works. That way it's a lot easier to install it. By the way disconnect the battery or your lights will stay on.
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Unscrew bezel and switch should slide out through the rear (under dash)
Check fuse Could be a bad switch
To replace the headlight switch in a 66 mustang, you first have to remove the instrument cluster housing. After you have that out of the way, you will see a small button on the housing of the switch mechanism itself. Pull the switch knob all the way out, as if turning on the headlights, then push that little button and the switch stalk will pull all the way out of the switch housing. Then unscrew the switch bezel, pull the switch housing back, so that it clears the dash, unplug that big wire connector from the side of the switch, and remove the switch entirely. To install the new switch, you just do what you just did, except do it backwards.
A plain Jane 1966 Corvette coupe sold new for $4,295.
The headlight switch may be at fault or the wiring has been altered.
I would guess the headlight dimmer switch on the floor board is bad or has a loose wire. Possibly the head light switch is bad. The headlight switch has a separate circuit breaker for just the head lights.
Just remove one screw inside car, under the dashboard near the ignifion switch area.
Bad ground or bad switch
The 1966 Chevy Impala ignition switch can be removed by loosening the retaining ring at the top of the ignition switch. Remove the wiring harness. Reverse the process to install the new ignition switch.
You should use 327ci 300hp 1966 Corvette engine oil ;)
Change the headlight switch. If the switch fails to fix the problem, you may want to pull the headlamps out and check the connection at the back. Lastly, inspect the complete headlight wiring harness for missing insulation, exposed or frayed wires, or loose firewall connection.