I don't think you need to have all of the grids functional just to get a continuity reading.

Nor did I! But I’m getting nothing.

Are you really getting nothing (OL or infinite) resistance, or are you getting "0" resistance? There's a difference. The defrost grid will be a very low resistance, it's fed with a 30A fuse meaning it's resistance value will be less than 1Ω. If you have a 200 scale on your meter, it's the one you should be using. If you truly have an open circuit, you have a main connection open somewhere. A broken trace or 5 won't cause an open circuit, just a higher resistance value.

Use a TEST LAMP for automotive circuit testing ! .

Apologies for the lack of clarity... not my realm. My multimeter shows a "1" when the probes aren't touching, and a resistance reading of... ".03" ohms or thereabouts whenever they are. Simple harbor freight job.

I'm getting 12v on the driver's side screw -good- but when I switch to Ohms and place the grounding probe on the passenger side screw, I get the standard 1.
I've tried using a few inches of element between the probes and still get a 1.
Either there's too much surface corrosion to make proper contact, or I'm really doing something wrong here.

If it is corroded enough that the circuit is already broken/open polishing it off won't restore the circuit.

Right, so this is a little odd...

See images:

http://i65.tinypic.com/2gyatfa.jpg

I cleaned a small area, about an inch long, on both the driver and passenger side rear window tabs.

Tested each area independently to make sure I'd get a resistance value –checked out. Cool.
Then tested each side linked together (probe on each side) but registered no value.
That seems extremely unlikely... Every element would have to be broken for this to be the case?

Nevermind resistance, I figured tracing voltage might be a better place to start.
I'm getting it at the screw (in photo), but not on the small tab I cleaned (?!?!).
Where does this line connect to the window?

Hmmm..... I only see one wire on that terminal. Which must be from the fuse side.

The window side must be nearby. Possibly tucked behind the interior panel?

Should there be another wire under that screw??

That would seem reasonable to me. One coming back from the fused side and one to the window.

I see no flaws in this logic.

On the passenger side is a single ground wire running from the tab to the chassis. Think I got lost in the symmetry.
The only thing left to peel back on the driver’s side is that insulation. With 38 year old glue holding it together, im a little reluctant to go exploring.
Any idea what he other visible wire (thin black) is responsible for?

Before pulling the sail pillar again, anyone else who’s seen the proper configuration care to chime in?

I was wondering the same idea. You may have to determine the possibility of where else could that wire connect
Maybe a slight tug. If that's it, there wouldn't be much holding it back if it's la loose end. I understand the hesitation. Hopefully someone chimes in with recent experience.

That thin black wire in your picture diving under the padding is your glass power lead. Carefully tug on it and see where it goes. If you pull it up and there is a free end see if it has continuity to the cleaned off area on the glass. If so it belongs on the screw with the car body wire. That black thing with a screw is just a terminal block.

My 123 did have breaks in every single wire going across. Apparently the conductive paint they used in that era had aluminum in it and it corroded and failed open circuit in multiple places per trace.

I would put the meter on ohms and start with one probe on your cleaned off area, carefully start probing along each trace and I think you will find that it will die off fairly quickly as you move away from the header on the side.

Put one of those bright LED work lights (don’t use a halogen, it’s too hot!) outside (or use the sun), and make yourself a slit screen out of a piece of thin opaque cardboard, to view only the line from inside (so you don’t get blinded), I bet you will find the breaks that way.

Unfortunately there is not too much you can do to fix this short of replacing the glass with new.

Hmm... not so encouraging.

I removed the sail panel again but no joy tugging the thin black wire. It’s definitely attached to something on either end. The upper end is still factory taped to the column and travels up into the headliner. The lower end I followed down through a hole in the Pillar and toward a place I can’t follow.

Any other ideas? I’m happy to attempt fixing the traces but I need power at the tabs first. Surely someone else in here knows what I’m looking for?

Well, in case anyone's still reading, found my way to the junkyard yesterday and managed to get eyes on a completely stripped 240D. I'm definitely missing a wire to the rear windscreen.
There's a wire on the left side tab that is the ground, but nothing on the right.

It appears I'm missing the tab and whatever wire is connected to it.
Question is whether or not I can mock one up?