Hi. Someone can correct me if I'm wrong, but my 1996 S500 (W140) Coupe only has one heater fan. Its available as an assembly, a motor and fan, or just then motor... so, that may be the confusion.
Phil at FASTLANE can get you just the motor... that's what's squealing... at least it was on mine. Mine squealed regardless of fan speed too.... sounded like like somebody strangling a squirrel in the dash.
Easy to get to... take out the 6 or 7 large phillips head screws that hold the air plenum (cowling) in place right in front of the windshield wipers. Once its loose, you'll have to remove a vacuum line and several sensors to get it completely free.
Once its out, the main cabin air filter is directly underneath on the passenger side. Lift out the filter and you're looking at the fan assembly. Comes right out after you remove the screws and turn it a bit. Easy job.
Hope this helps. Good luck.