why can't i unlock my grand marquis with the key.

It would be helpful to have a bit more information! Does the key work on one door but not the other? Did it ever work (as far as u know)? Does the key turn & not unlock the door or does it not turn at all!

If the key turns the lock, but it does not unlock the car, most likely the rod that goes from the lock to the door latch has come loose. It could have happened when someone tried to use a ''slim-jim'' to get into the car. In that case, u have to take the door panel off & re-attach the rod!

If the key does not turn, either the lock cylinder has a sticking tumbler! Try spraying WD-40 into the lock to see if it will come free! If that does not work, u will need to have the cylinder replaced. Any good locksmith can key the replacement cylinder to ur current key!

If both doors do not unlock with the key, & they did before, something is wrong. Either u have a wrong key, r looking at the wrong car, or the key was damaged in some way .

l have a marquis with the same problem. More than likely you always use the electric door switch or remote to unlock the doors. This causes the key part of the switch which is never used to corrode. You have to pull the door panels and lubricate the key assembly and the rod assembly .