Okay, got some links for you that may be helpful.
FreeSSM. An open source Select Monitor program. Maybe you can learn something from the source code?
General SSM info. It's on the 2.5RS forums so everything revolves around the Impreza but it's a pretty safe bet that the changes are applicable across the lineup, generally speaking.
More info on SSM1 from a dedicated SVX site. LOADS of information on here for the pre-96 models.
Hope this can help somehow!