Thanks, M.B. Doc. With your guidance, I'll take the cluster (back) out and hunt some more for that ground so I can fix my flaky wipers and cruise control.
But an update on the dead power windows is in order for folks who find this thread with a "search". I had previously narrowed my suspects to the G102 ground or the Power Window relay. Having given up on finding G102, I went looking for the relay, which my 107 Service Manual said (mistakenly, it turns out) was under the Left Hand Kick Panel.
Because of the manual, I ignored the two shiny metal relays under the Right Hand Kick Panel (over the fuse box), saying to myself, "no, that couldn't be it because there are two matching relays, not one, and its on the wrong side". So today I stopped at All Mercedes, picked up a new "Window Regulator Relay(?)" -- noted it was a black plastic relay with flat stake terminals not round pins like the metal ones had, and with renewed confidence, tore the left hand side of my car apart looking for anything like that new plastic relay. Nothing. There's no wiring under that panel at all. Finally, before I buttoned things up for the night, I said, "The heck with it... I'm going to swap those two identical shiny metal relays over on the right side over the fuse box even though they're on the wrong side according to the book and don't look anything like the $26 widget I just bought".
Presto. Working windows. One of those two relays must be the window relay (heaven knows what the other one is). So tommorow I'll go trade the wrong part I bought for the right part I need and away I'll go with operable windows.
I did manage to follow those brown wires to the right of my glove box I mentioned in my last post and sure enough, by removing the underdash panel on the right side, I found that like 10 of them all went to two big grounding screws screwed into the bulkhead. Nice and clean and tight, so that wasn't my flaky ground. I thought sure I'd found the elusive G102 there until I saw MB Doc's authoritative reply.