Adding an extra PK3M key to 2000 Grand Prix.

I have a 3.1 liter, 2000 Grand Prix with 125,000 miles on it. The key that works right now is a PK3M key except the rubber has fallen all apart. I have another PK3M key, but it only unlocks the door. Whenever I turn the ignition on the Security light comes on and when it turns off when I put the car in gear. I know there's a transmitter in the key and it has to be programmed, but how exactly do I do this?

Start the car for a minute with the working key. Then shut it off and immediatly put in the other key and start the car.

It will only work if PK3 is stamped on the key.

Okay, I will try that. Yes, both keys have PK3 stamped on them with a M underneath.

Did you imply that the additional key won't work if it says PK3M? I thought that might be why I'm having trouble since it's two master keys. I tried what you said, but the security light stayed on solid while I was in Park. I was able to drive, though, which was weird. I'm not too sure if I want to switch to that key with the security light staying on. Should I go buy a PK3 key with no M on it?

I also tried this fro my manual:
To program the new key:
1. Insert the black master ignition key in the ignition
and start the engine. If the engine will not start, see
your dealer for service.
2. After the engine has started, turn the key to OFF, and
remove the key.
3. Insert the key to be programmed and turn it to RUN
within 5 seconds of removing the original key.
4. The SECURITY light will turn off once the key has
been programmed. It may not be apparent that the
SECURITY light went on due to how quickly the
key is programmed.
5. Repeat Steps 1 through 4 if additional keys are to
be programmed.

Hmmm, interesting. If it has PK3M stamped on it, it should work. Was the non working key original with the car? Maybe the transponder in the key is bad. I purchased a new transponder key off the net for my car, and programmed it with the above method and had no problems. Try starting your car with the good key for only 30 seconds, then shut it off, and within 5 seconds start it with other key.

No, it's not the original with the car. My dad purchased it from a local locksmith when he first got the car. He said the locksmith programmed it, but the next it didn't work. I'm going to go try what you suggested to see if I have any luck.