Peter,
I think that a PO has maybe tried to fit 4 way hazard flashers at some time.
I have a diagram for such a fit, that I can probably post over the weekend if you want.
Regarding the Washer function, mine is also inoperative from the stalk,
so I have fitted a small chrome momentary push button switch (from Maplin)
under the right side of the dash.
On my attached diagram here, you link the Switched Live 8, to the Washer 11, via the Maplin Push button.
(your 280SL may have different connections to my early 250SL)
Good luck
Paul