I cleaned my battery connections again even though they looked good, and got a .5 volt improvement at a couple of points along the electrical path between my ignition switch and my starter (under full solenoid load - I disconnected the red wires at the starter and the primary coil wire at the distributor). The solenoid clicked very loadly right after doing that. Unfortunately the starter motor would not crank anymore.

In short, my starter turned out to be bad. It was a rebuilt 8 year old Bosch.

The positive block you mentioned may just be on the bottom side of the relay mounting chasis which is the same piece of plastic which is the fuse box. There I discovered a copper strip onto which several large red wires attach.

An important bonus was that after cleaning the battery terminals, my AC now works again. I'm guessing the voltage is now high enough so that the AC clutch engages, where as it didn't before.