I got part of the dash working, so I thought I would start to figure out how the get the MIL or check engine light to work. Come to find out its not that straight forward. On my 1982, the ECM sends a MIL signal to a light driver module which in turn sends a voltage to the MIL bulb to turn it on. On the LS1, the PCM sends a MIL signal to the BCM, which in turn sends serial data over the the LS1 gauge cluster which then interprets the signal and turns on the MIL bulb. I am stumped here, what did you guys do ?

Your LS1 sounds very different from my LS1, I just connected the PCM MIL output to my in-dash Check Engine light and it works as designed.

I'm running an '01 LS1, PCM, and harness...no BCM at all.

Same here, there was a brown/white wire that came from the instrument cluster and went to the connector for the old computer. I cut this wire and sent it to my new LS1 computer and it works just fine. But this was on my 89 IROC and i'm sure it's different on your 82, I'm trying to find a schematic on Austinthirdgen.org and it's not showing SES light in their diagrams. The SES or MIL has 12 volts in run/start and the PCM is what grounds it. I'll try to look around some more and see if I can find a diagram.

The light driver module was on older cars... I had this on my '85 iroc as well and it will cause the output of the ECM to be inversed if you connect to it... ie your new ECM off will cause the MIL to be on, and ECM MIL on, will cause dash MIL light off

The light driver module is behind the dash pretty much behind the radio / vent controls... it is taped into the harness on the '85... what i did was simply cut it out and tied together the input wire to the module to the output wire (i had the input wire being driven by my new ECM as i brought it in and mounted it in the same physical space of the old ECM.

it is as simple as that