Do you have a red warning light on the center gauge stack that's illuminated "Headlights" or the like? The '69 I own has one (but I can't tell you the exact verbiage, and it's cold out there...)
It's to tell you if you pull the light switch, and the vacuum cans don't fully move the headlights to "Up" and locked position. If they fully deploy, and the warning light is on, there are two safety switches (one on each side) on the headlight lift mechanisms. The buttons need to be fully pressed in to turn the warning light/buzzer off. They may need adjustment (it's two nuts, on a threaded shaft.)
Or after 40 years, it's possible one of the switches has failed, wires corroded, or have wiggled loose. It's also possible that the vacuum motors are getting weak, and they're *not* going up all the way (pretty common.)
Before you tear into a C3 dash (which is unpleasant, to say the least) check for the warning lamp, and those switches. Should be able to wiggle under the air dam and see them pretty clearly.... Parts are reproduced for those switches, vacuum motors, and harness parts.
You may need a multimeter, and some choice words for the GM engineers who decided vacuum lift motors were a great idea....