I would suggest using a separate switch. If it always comes on with the headlights, you might go nuts with the ambient light being on all night long. Or depending on where you drive, you might get pulled over for bright or certain colors of ambient light (blue or red specifically).
You can find ambient light kits sold on amazon. Google Ambient light or 12 volt ambient light kits.
Example I found on amazon. I have no info other than finding it on amazon and the reviews it has.