The switches were all set up for flash-to-pass, but the feature did not work on many 230SLs destined for the U.S. market. Mine didn't when I acquired it in 2003. However somewhere on this site (or one of its predecessors) there were simple instructions involving doing something with a taped wire on the rear of the fuse block. I don't remember exactly the procedure (it was a couple of decades ago) but my flash to pass has worked ever since.
There is something on this in the Technical Manual under Electrical/Lights/Light switch, then scroll down to Flash to Pass.