I traded stocks 10 years ago and I can say that they do have impact on forex.
In Murphy's first book on Intermarket Analysis, he made a strong case for sustainable market relationships. Years later he published his second book on Intermarket Analysis where he introduced the term "decoupling".John Murphy; Intermarket Analysis : profiting from global market relationships. Some good examples, old book get it used low price good info
If it holds useful anlysis info it could be used as factor in strategy.Correlation is a useful analysis technique but it isn't a trading strategy by itself.
I think you summed it up as shown above. Any keen insights on relationships are likely to be proprietary and not published. Best approach is probably your own research.If it holds useful anlysis info it could be used as factor in strategy ... but most of the relations seem to be in constant change and not much consistency.