Spent this evening crawling all over this dash. If there is supposed to be a ground at the the mid-structure, I'm not seeing it. At all.
I'll assume you were also looking for it just hanging in the breeze. At this point ya may just have to run the length of harness and look for a single black wire coming out of it.
EDIT - You said that cluster grounds at black wire at the 6-pin connector (CI 5, H4 20BK). Tracing that ground from the panel ground, it takes me to the cluster illumination lamps, the ash tray lamp, the heater panel lamp, the clock, the seat belt lamp in the dash, and the map light...all of which function, as do both turn signal bulbs and the hi-beam indicator. Looking at the three inop gauges, I see the shared ground via the voltage limiter...which means that if the rest of the cluster is working and grounded, what's the issue here? This leaves me two more points to look at:
Not sure if you're chasing that in the book or in the car so: ohm meter from connector hood in the harness to any convenient ground point will tell you if CI 34 is attached somewhere. I'd feel better laying hands on it.
* CI 4, 20DGN - This is the power wire to the radio.
*The output point at the voltage limiter to the three powered gauges.
Since the radio functions just fine, this leaves me no other option than the power point at the voltage limiter on the instrument cluster, right? Could the ...what is that, a condensor?...be the issue, or would I be looking at a break in the panel? I'll plug everything in next time and see what kind of voltage I get across the gauge points, but I think I have my target zone now.
CI 4 20 DGN is power to the cluster coming from fuse #8 (zone 83D) which also powers the wipers, cruise control, rear defrost. The radio is on a different fuse and doesn't connect to the cluster but here's an idea: Cluster on the bench, use a couple small jumpers to hook 12 vdc and ground to the ckt board at the connector points, look for 5v pulse on the output side. The condenser is to filter out the limiter clicks. Wasn't step 1 in all this replacing the limiter?
