If you take the car to a dealer they can plug it in to a scan tool and get the codes by calling into GM and inputting them into the stereo for you[was no cost when I asked]. The code is made up from the vehicles VIN. I am assuming they plug in the scan tool to verify the radio is from the car it is in and not stolen.
Use a 1999 grand marquis gs key!!
You will need to call the dealership and give them the vehicle identification number.
In Stereo was created in 1999.
Bring Your Own Stereo was created in 1999.
Strange in Stereo was created on 1999-04-20.
Pumping on Your Stereo was created on 1999-05-24.
I guess take radio out and buy a better and cheaper radio
See if unhooking the battery terminals for a few minutes doesn't reset the system and allow access to the radio.
To remove the stereo off of a 1999 Honda Civic loosen all of the mounting screws for the center console. Pull out the console is loosen the stereo mounting screws. With everything off pull the stereo and unplug the radio.
If you are asking about how to get a stock stereo to unlock it's own security system once you've dicsonnected and re-connected the battery, then I recommend that you stop by any GM dealership. It takes them about thirty seconds to get the code that they need and about thirty more to get the stereo working again. It's usually free, why not let the pros do it?
A Sledgehammer
I have a 2003 grand prix GT and the unlock code is 1040. use the hours and minutes and then press AM/FM. wait and it should work.