Mmm.. sounds like two bursts of serial bitstreams, then back to idle levels. This will NOT be as simple as just connecting it to the mute input, since it doesn't stay in the lower voltage state after sending it's "message". The slightly lower voltage "seen" by your meter is just an averaging of ones and zeros being transmitted rapidly.

So a software decode might just do the trick.. just gotta ensure we poll every 500us or faster, so we don't miss any of the 1ms bits (1000bps somebody said..).

If you can hook up your onstar and empeg together, and fix the in-car baud rate for the "normal" serial port so that a laptop can be used (the silly software switches from 115200 to 4800bps midway through initialization, but only in car mode.. ugh..), and then hook up a laptop to capture serial output.. then I just might patch hijack for crude bitstream interception to see if we can decode anything from your OnStar unit.

??