1991 is the only year for the W126 350SD. Mine has heated windshield washer system too, but the heating in the tank is done with a heating coil from the heating circuit and the nozzles are heated electrically just like the 1986+ models. Yours may have some sort of optional feature that isn't documented in the published FSM.

The Gen II (1986+) W126 didn't change much during its life. The ETM usually shows when things changed and is typically marked such as "From 1988". Other than the Klima relay and the EDS/Tach system, the wiring is much the same between gas and diesel cars.

There is supposed to be a FSM supplement that was printed media only that covers some of the 1991 modifications. If you have a car with a lot of features, it may be to your benefit to hunt one down or call the Classic Center to see if they can procure a copy for you.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]