Well on that year w124, the windows are fused on opposite corners. The G position is for left front and right rear. Take another look at the fuse schematic on the inside of the cover. Unless they have changed it, should say window lift left front a right rear or something like that. I beleive they designed the circuits that way to avoid heavy current loads on one circuit, ie lowering both windows on the same side at the same time. Anyway, if G is blowing when you turn the key, the right rear should not be working.
But, you say that it is by using the rear door actuator button. If your fuse schematic says there is only one fuse per window, try and identify the other three. I don't think you will find them. Don't think it's a voltage problem. There are other systems in the car that would take a hit if that were the case. I'll look in my manual of schematics to see if there is a relay involved, (think there is).

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.