Not sure about an 1198, but there is a security bolt hidden within the outer ring to prevent theft on the earlier models, so I would suggest getting a locksmith to pick it.I haven't had an OEM cap in a long. long. time, so I may not remember perfectly.
You can remove the screws that hold it in, and remove the entire fixture, and perhaps work on it once it's out. Press the release from under and lift the lock mechanism out to take the key out? You could take it to a locksmith and see if he can jimmy it out with his magic touch? You could drill the shit out of the opening and destroy it You could just replace it with a non-keyed cap, like a lot of people do.
you can buy a used OEM one and have a locksmith key it to your current key? just thinking out loud here for a few options.