Sounds like something is not getting enough current, possibly due to a corroded fuse or fuse contacts. When the climate control blower switch puts the blower in the higher speeds, so much current is drawn that the voltage drops. Anything else on that circuit (if any) would then malfunction.

More information, please. When the (climate control?) Auto or High buttons are NOT pressed, does the (left or right?) rear window operate normally? If the window does operate normally under any circumstances (a) does it work with both front and rear switches? and (b) you don't need a new regulator.

The buzzing noise "near the trunk" -- can you hear it inside the passenger compartment, in the trunk, or both, and is it continuous?

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970