There's got to be a " CHIME RELAY " out here that will work, like the one that goes ding-dong when you leave the keys in the ignition and open the door, or the lights on...
The people that supply re-wiring for our restoring jobs, will come-up with something for 6 volts .... if we-all ask. .... as for your wife, you could put it on her side of the car, and tell her " someones at the door....dear... " hee...hee...he.... OLD...BILL