A possible alternative fix to gain the Onstar mute function:

Apparently the SWI-X works with 1989-1993 pontiacs that utilized the E&C circuit (the same one that Onstar mute uses). It might be possible to interface the yellow wire of the SWI-X (12V serial data in) and convert to IF (program interface by pushing Onstar button then remote mute or aux. button). Thought I'd mention this if anyone wants to try it.