Thanks, Gabus. Sounds like I need to check continuity between the outputs of the relays and the metal strips in the picture. Can these be fixed by re-heating the solder? Or by placing a wire between the output and the strip?
Are the relays replaceable or do I have to replace the whole board?
MB Service Manager suggest inspecting the turn signal bulbs themselves. Says that if the two silver contacts have worn or melted together that the resulting short will cause the flasher to fail in the "on" state.