A trip to the dealer$hip was very informative.
The wire from the alt is the single one that leads to the center pole of the 3-pole connector. that wire seems fine - i am hoping the low voltage is due to the shorting at the 3-pole connector preventing the current from making back to the battery.
The 4 strand wire is the harness for the ACC. It also connects to the drier, compressor, and the fuse box. Not indicated in the schematic, but it must be #8.
Cost of the harness alone (no connector included) = $875
So I called a salvage yard and they will sell me both the harness and connector for $100.
So I will head there later this week and get a harness. I imagine it will be a bit of a bear to rewire that. Hopefully I can get to all the places I need with the dash still in place, but we will see.
Soooo...that part is one of those things that MB contracts to unicorns, and replacement with a new harness is almost not worth the value of the car - considering that I am not sure why it melted in the first place, and whether the replacement will do the same!
Has anyone here replaced this harness before? Haven't found any links in searches.