I just wanted to post that when I bought my '84 300 SD almost 2 years ago it had two non-working windows (the driver's and the driver's side rear). The driver's was a bad motor that jbaj was able to fix (thanks Bruce!). The rear was a bad wire that I just was able to diagnose and (in the process) I learned about how the switch works and which wires do what. Now all 4 windows work (at least for the time being

).
I credit my success to this forum. Thanks to everyone who has helped along the way (just by me reading old posts in many cases).
If anyone would like specifics on the rear window switch and the wires and which carry what voltage for each switch setting, please let me know and I will post that information (if it is not already common knowledge).
Dan