I've got the same thing going on with my '82 300D. Its either the return hoses or the injectors themselves leaking. It got worse suddenly when I ran a full bottle of Red Line DFC in a tank - maybe it cleaned out something that was plugging the leak.
So the easiest thing to fix first is the return lines.
I've been looking at the wet areas, trying to decide if its the injector or the return hose. Can't tell for sure, but it looks like the whole injector is wet - leading me to believe its the return hose which attaches at the top.