I think this is going to be a problem. Your 1999 DME is a 5.2.2, and I do not think it is capable of dealing with the code for the 996 as it was a pretty basic DME. If it can be done, you will need the car's codes and access to a PST II or PIWIS unit and a cooperative dealer (they are not supposed to help people do this).
You might want to pose this question to one or more of the aftermarket software guys, they may have something you can use instead.
