I'm a big fan of using the pull function on the turn signal indicator to go in and out of OD. This requires a relay, as the pull function is only momentary.

I have mounted the standard red or blue light in a piece of stainless with a radiused right angle bend that is mounted to the bottom of the dash using the two screws that affix the q bottom of the heater controls. I can send you a picture if you'd like. Easy to fabricate. And no unsightly hole in the dash.

1967 and later (maybe earlier cars have this extra unused wire) 122s have a (pull function grey) wire provided, but unused in US delivered cars taped into the harness under the hood on teh driver's side, and easily accessible to connect to a relay. In Europe this was used as a high beam flasher, not legal in the US at the time.