1999 Sable wagon - the dash and rear lights don't work. Brake lights and headlights are fine. Any ideas?

yes ,
check the fuses.

Headlight switch is what it sounds like.

My first guess would be a ground problem.
The headlights and tail lights are controlled by the same headlight switch, but the brake lights are on a totally different circuit.
You may have to get a shop manual to get the wiring diagrams needed to troubleshoot the circuits if the ground connections are ok.
The lights involve the body computer and other components.

These are all different circuits. The brake lights are on a 10amp fuse - the headlights are on a 50amp fuse - and the parking and dash lights are on a 20amp fuse.
Do you have power going into the headlight switch and the brake light switch ?

as for the reverse lights if the bulbs are good and they are getting power it sounds like your backup switch in the transmission went out

Most certainly a wiring fault, you need a wiring diagram, these items are related as in all on same circuit. Check rear of fuse box.

Try test the Mega alternator/ battery fuse, should have power across both circuits.

Assuming that you have checked and replaced the brake light bulbs if necessary then, if your dash lights are also out you should change the headlight switch, if you have dash lights the check the brake light switch (under the dash).

If your dash lights are out also ,the dash dimmer switch is bad