Thought I had only gotten one of two keys for mine, that it was a two key system. Instead, the original owner never used the keys...opened the doors with the fob and/or the door keypad, and the trunk via the button on the driver door. Had to use a bit of WD40 and perhaps some PB Rust Buster (don't remember if I needed that or not), being careful to wipe off immediately to protect the paint.
After a couple of cycles, the same key that worked the ignition worked both the door locks and the trunk lock.
Apparently this is a common problem with original owner used cars, especially if they are "grandpa" cars.
The lubrication of the locks in the doors and trunks loosened up the cylinders, and the keys I had, the same ones that went in the ignition, began to work.
Seems like it is an easy thing to have happen if your car had an O-O (Original Owner) who never used the keys for the door or trunk, since they aren't needed, given a key fob that opens both, plus a door keypad and a trunk release button on the driver door.
I seldom use the key either for doors or trunk but do use them every once and a while just to keep the lock cylinders from seizing up again.
Hope this helps...
PS, when and if your 97 GM got an aluminum intake manifold to replace the partially plastic one, and if you don't have ball joint problems, or get them fixed, you have a great used car with the potential for some serious almost trouble free longevity. Mileage not bad for a V8, and a lot of fun to drive, plus comfortable for six adults for a long trip.
One of the last of the great American rear wheel drive, body on frame, V8 cars, with overhead cam and sway bars front and rear, even on the base model, thrown in. Read somewhere that a new owner showed his base model to an older relative, who thought he had gotten one that was loaded with optional equipment. Although Ford did do some "decontenting" in later years, especially in its sister car the Ford Crown Victoria, overall the car comes with a lot of really nice things as standard equipment.
I will drive and enjoy mine for years, if I last that long, as the car is quite capable of around 400K miles before needing major repairs, and mine arrived with a good bit less than 200K, and drives like new. Among other cars I have owned were a five year old 1961 Jaguar Mk II 3.8L sedan, and a brand new 5.0L 1988 Ford Thunderbird with leather seats, a moonroof, billet aluminum wheels, etc...capable of 143mph top on a closed circuit, which I got to prove. But the Grand Marquis is as much fun for me to drive as they were, though in a different way. Guess I'm turning into a geezer, and some people say it is a geezer car, but it is the hottest geezer car I've ever seen or driven. A great car all around.
Ford has made a few engineering mistakes, including with this car when it was first built. But once you get it straightened out, the only cars that can come close to touching it cost literally dozens of times more, perhaps even hundreds of times more, than you can get one of these for in good condition.
PS You should have a second type of key, but it is a valet key...only operates doors and ignition. You can use one of the regular keys to lock the glove box and the pushbutton for the trunk, that is on the driver door, and the person with the valet key will be able to lock and unlock your car and drive it, but cannot get into either your glove box or trunk. If you don't have that key, you might be able to find the key code from Ford Customer Service and get a new one cut. Great if you have to park in garages but can't park and lock.
