In the process of troubleshooting the rattling noise in the '90 SSE, I pulled the alternator from the '91 LE to see if the noise followed the alternator (it didn't - turned out to be the harmonic damper on the '90). I put the '91 back together the way it was, started it up to verify it was back together properly, and went back to the '90.

This evening, the '91 wouldn't turn over. It just buzzed & clicked a little when the ignition was turned to the start position. That was traced to a bad ground connection at the battery, the solution was to use the parallel negative post on the battery to connect another ground cable to the engine.

Now, it turns over but won't start. Has fuel pressure and sparks, but won't fire. I'm assuming it isn't firing the injectors, because the plug I pulled was dry. After checking all fuses, I realized the SES light was either dim or not on at all. Definitely not on while cranking. I changed the ECM fuse just to be sure, no dice.

So, think I fried the ECM by attempting to crank it with a bad ground? Where is the ECM, anyway? Is there a fuse other than in the fuse box, or a fusible link somewhere?

This may be the last straw for the car. It was rear ended and totalled in 2000, but we've kept driving it with the crumpled trunk and universal taillight. The struts are shot, it was caught in a hail storm 2 years ago, the driver'* window doesn't go up after going down, etc., etc., etc. Another $80 for an ECM is hard to swallow. But, the engine and transmission are sound, so they might end up in the SSE and the '91 junked.

On my car the ground cable goes elsewhere besides the engine block. I may be reading your post wrong, but it sounds like you have a battery with side and top posts and you hooked up a top post cable from the battery to the engine. Correct so far?
If so, then the rest of the ground connections that feed the computer, etc. are still hooked up to the battery via the corroded side terminal. Even if the ground is good from the battery to the block, it doesn't necessarily mean that it will be good back to the brain, assuming it is still connected with the old terminal.
To simplify, Make sure that all the ground cables attatch to the battery using the fresh clean terminal, otherwise, the computer and other stuff won't have a complete circuit.

Sorry to hear about the wreck. I always wondered what one of these drivetrains would be like in a VW type off road chassis.

True that there is a ground from the battery cable to the chassis (fender) via a small wire. I thought about that, but the 2nd battery cable (and you're right, the battery has both side terminals and top posts - I added a 2nd cable to the top post but the side terminal cable is still there) is providing the ground path back through the original cable to the fender.

The "new" cable has a small wire coming off of it as well. I didn't take the time to connect it to the fender - I'll try that and see if anything changes. I doubt it will, but can't hurt.

We had a member that had the same thing happen when pulling the alt.
Turn out the starter went bad possibly from the clean terminals and extra volts.
We had him rap on the starter with a hammer handle and it started indicating a shorted starter armeteur.

The starter works fine now.

The SES light is the only one that is dim.

You would think since the starter draws the highest load that if the terminals flow enough juice to run the starter motor, everything else must be getting plenty.
Not so.
I've seen a car that would start and idle for a few minutes and then suddenly start a wild surge/ cut out routine before it would finally die. There were no outwardly visible signs of corrosion on the terminals. Only a bulge toward the end of the cables. I cut the insulation off the end of the cable where it was bulging and it was green from corrosion. I solved that problem by cutting back the cable to good copper colored wire and installed new terminals. Car ran like new.
So just because it starts is no certain indication of a clean circuit path to the ECM.

Connected the small ground wire to the chassis. No different, still won't start. Turns over fine, no SES light.

Did you check all the fuses?
Do you have a voltmeter or test light?

I checked all the fuses. I determined the location of the faulty ground with a voltmeter.