Why won't the tail lights work on my '94 gran marquis when fuse and bulbs are good?

Why won't the tail lights work on my '94 gran marquis when fuse and bulbs are good?
First step would be to check for power coming from the headlamp switch (BR wire).
There is a connector that leads to the rear of the vehicle that could become corroded.
Make certain that you have both power and a good ground circuit to the rear sockets.
Has this ever been equipped with a tow harness? That could also contribute to a damaged wiring harness and/or corrosion.