Hi John
A couple of points:
Have you looked in the Technical manual here , under Steering Column Switch ?
The Multi Pin connector should have 12 pins, I think , not 6
My drawings are for an early 250SL RHD, so your 280SL will be different, as the Column switches are not the same.
It is best to identify all the cables on the multi pin connector under the dash and check they operate on the wiper Relay connector under the "hood"
You can unplug the Wiper Relay, and check that it operates by apply 12 volts to the coil and seeing that the appropiate contacts make and break.
Hope this helps, I cant be more specific as Your wiring will be different to mine.
Let us know how you get on.
Keep safe
Paul