Interior Light Time Delay

your 68 could have gotten a lighting group addition with delay timer and lighted ignition switch bezel. The lamps would stay on for a time or go off when ignition switch turned to on position. The light didn't drift to on/off like the modern electronics.
I think the OEM wiring and parts would be the proper way to add the feature but be forewarned... that delay timer is known to fail, holding the lights on, killing the battery.