Well sure it's hard to get to on a 126,it's hard to get at on all cars in general and MB is certainly no exception. It's located just inside the firewall in the heater/AC box, the dash needs to be removed to remove the heater/AC box.
On the transmission, start with checking the fluid level (always!) and then move on to the throttle valve adjustment. This is a 722.5 5 speed transmission, which is unique to 300SL's and 300SE and SEL's starting in 1990(140 and 129 chassis only). The "nudging" was possibly just downshifting the transmission, this may be normal operation. I believe you'll see a "4-D" symbol on the shifter which indicates 4th gear or Drive selection. Any idea when the trans was last serviced? A fluid and filter change is due every 30,000 miles.
On the rear window, the regulator likely is broken.