Ron,
thanks for the latest info, it was all good. I expected to be chasing GMLAN wiring, and I may yet need to before all is said and done, but I found ECM, fuse #14 - DSL ECM (25A) does not receive 12volts with any position of the Ignition Switch, initially, about 0.1vdc. Fuse #13, ECM-BATT...