If both are inoperative check the fuse, and the brake light switch.
Under the hood. Fuse number 69 is the tail light fuse
If the tail lights on a Ford Expedition are inoperative, it is likely there is an electrical issue. If all the tail lights are not working, it is likely a blown fuse.
Make sure the bulbs are inserted correctly, if o/k look for wiring issues. A common cause is a recent trailer harness installation or repair.
Tail lites work, do flashers work ? Is the bulb good ? Try a new bulb first, still does not work. Check the wire just before the lites for power with a 12 volt tester, if power check contacts in the bulb holder. Still does not work and no power to the lites, you will have to find the right wire up in the front of the car and check there, could be a break in the line.
If tail lights and dash lights work - check bulbs
Couple of things. Check the ground for the loom at the rear. If that is clean and tight I would suspect that the problem is in the brake light switch.
check the brake light switch by the pedal. it might be kind of underneith the dash it might just need to adjusted
did you check your fuses. Has ther been any repairs back there like a new fuel pump? Is it four wheel drive and someone snagged a wire? I would say it is an open line or the possibility that both tailights went out and you just now noticed. Check the bulbs
Dash lights and tail lights usually work off the same fuse. Some vehicles have two fuse panels, one inside usually under the dashboard and another inder the hood. If the fuse is good, the problem may be a bad connection or burnt wire at the fuse block. "Jump" the leads where the fuse fits into. If you still have no lights, there is a bad connection or burnt wire. If you get lights, the fuse neds to be replaced.
If the back dash light will light when the brakes are applied but the two tail lights will not light up, there is probably a short in the wiring somewhere. It could also be that the fuse for the brake lights is burned out.
Open rear tail gate and remove lower half of panel. You will have to remove 4 screws. Then it will snap off. You will be able to reach inside of the metal frame work to reach lites.
You have a short between the 2 circuits, check at the rear bulb sockets & see if a trailer harness was installed as they usually are the source of a lot of bad wiring