Here’s what I did when I installed two lights under the dash. They are activated by door switches which do not ground. They simply complete the circuit to the grounded bulbs. I also put an override switch under the dash in one of those holes under the steering column. There are two routes for power to the new bulbs. It can come thru the green wire in the diagram from the existing dome light wire at the headlight switch, or thru the blue wire which goes thru the override switch to the door switches. It’s a simple add on harness using the headlight switch connections.
When you turn the dome light on with the headlight switch, or open a door, all the lights come on. If you want to leave a door open you can turn the lights off with the override switch.