1995 Mercury Sable head lights

When you turn the headlight switch to on, that only controls the headlights, not the interior overhead lamps. But if your talking about the dash lights turning on as opposed to the interior lights as it sounds from your question, then chances are good that wires leading up to the dash panel are broken.

Generally the wires as behind the dash, and depending upon the year of the car, can be a pill to tackle on your own if you've never done it before.

As far as the rear lights go. Check for bad bulbs and/or sockets. Given the age of the car, chances are good that that would be the best place to start.