Driver Power seat
If it goes all other directions then I would suspect the switch is haywire.
get in there and apply voltage to the other direction as tennisshoes suggests should go in two directions...reverse the polarity on the wires (with the battery disconnected of course)....will lead to a bad switch as tennisshoes has asserted~
If the failure has anything to do with the motor or gears in the actuator, I suggest a Wrecking Yard replacement of the entire seat mechanism. Dealer costs are very high and USUALLY (not always) the sell the entire assembly instead of replacement parts - probably $400 or more.
Looking for a Used Grand Marquis in your area?
CarGurus has 1,726 nationwide Grand Marquis listings starting at $800.
Search Mercury Grand Marquis Questions
Mercury Grand Marquis Experts