One additional note... I was unable to reproduce the 0.7V reading at B2 with the lights in the park position. All readings at B2 (in park and on) and R1 (in all positions) read 0.00V with the multimeter set on the 20V scale. On the 2V scale they read 0.001V.
The test POSITIVELY CONFIRMS there is a high resistance connection between fuse F1.10 and the main head light switch B2 terminal. The wire color is TN/WH. The confirmation is the voltage at B2 dropping to 0 volts when a load is applied.
Please look at diagram-92-1. Notice there is only one connector (C134) the circuit passes through. Assuming that there aren't any burnt/melted wires on the head light switch itself, the problem HAS to be in C134 or the Battery Junction Box.
Let's focus on C134. The service manual lists the location as "engine compartment, LH side, rear". The TN/WH wire goes thru pin #18.
Take the connector C134 apart. Look for bent/pushed pins. Evidence of overheating, corrision. Clean both sides with electrical contact cleaner. Apply a small amount of di-electric silcone grease to both sides (same kind as used on spark plug boots).
Next, repeat this on the battery junction box.
If no problem is found, then either the bad connection has been overlooked or there is a melted wire inside the harness. This will require ringing the circuit from C134 both directions to narrow down the high resistance wire.
IMO, the problem is not in the wiring harness. It is going to be in a connector. Short of rodent activity or wire chaffing, the odds favor a bad connector. I suspect the answer will ultimately be water in a connection.
C134 is the connector that goes through the firewall. It looks like the only way to access it from the engine compartment is by taking off the fender liner over the left front tire. The prospect of doing this wasn't that exciting. So then I crawled under the dash to see if I could disconnect it from that side. I wasn't able to get it off with just my fingers alone, so I got up to grab some tools and saw that the lights were all back on (I had left the switch in the park position). Sweet! So, looks like a loose connection in that connector. I still need to find a way to get the connector off and take a look at what the actual problem is, but for now all the lights work correctly. I must have jostled it just enough to get the bad connection back in place.
Thank you VERY much wmburns and nov8or! You guys were a huge help and it is much appreciated! If either of you ever makes your way up to VT, drop me a line and the drinks are on me!