There's a circuit comin' through the in-dash fuse panel that feeds the ECM. It should be labeled 'ECM'. I think it's fuse #32 or #33.
Somewhere in this thread:
http://forums.corvetteforum.com/show....php?t=2069247
there's some pics of my PCM with backprobes, for testing, along with some schematics, which I'm sure are identical for any GM. (circuit #'s anyway). I can't link the pics since I'm banned, so you're gonna' have to register to see the backprobe pics.