Re: 66 chevelle front marker lights.
Here is a way to make the lights stay on when you turn the light switch on if you choose.
Starting in 1968, when side-marker lights were introduced, the parking lights remained on when the headlights were switched on. Prior to that model year,the parking lights switched off when the headlights were switched on. I believe this pertains to all GM vehicles.
Disconnect the battery first. Then disconnect the light switch from the dash, and drop it down below the edge of the dash. Before you unplug the wiring harness from the switch, look at the position of the purple wire in the connector. It will be in a slot which is at the end of the switch, farthest from the dash.
Look for an open slot in the connector which is at the other end of the connector, towards the dash. Remove the connector from the light switch, and then release the terminal of the purple wire from the connector using a small screwdriver (you do this from the light switch side of the connector).
Insert the terminal of the purple wire into the open slot of the connector which you identified earlier (the one closer to the dash). Then just reconnect the wiring harness
to the light switch, and reinstall the light switch into the dash, and reconnect the battery. Now the
parking lights will function as before in the halfway position, but remain on when the headlights are switched on.